diff --git a/akka-stream-tests/src/test/scala/akka/stream/FusingSpec.scala b/akka-stream-tests/src/test/scala/akka/stream/FusingSpec.scala
index d8668937eb..84f4abd3d3 100644
--- a/akka-stream-tests/src/test/scala/akka/stream/FusingSpec.scala
+++ b/akka-stream-tests/src/test/scala/akka/stream/FusingSpec.scala
@@ -11,11 +11,16 @@ import akka.stream.Attributes._
 import akka.stream.Fusing.FusedGraph
 import scala.annotation.tailrec
 import akka.stream.impl.StreamLayout.Module
+import org.scalatest.concurrent.ScalaFutures
+import scala.concurrent.duration._
+import akka.stream.impl.fusing.GraphInterpreter
+import akka.event.BusLogging
 
-class FusingSpec extends AkkaSpec with ConversionCheckedTripleEquals {
+class FusingSpec extends AkkaSpec with ScalaFutures with ConversionCheckedTripleEquals {
 
   final val Debug = false
   implicit val materializer = ActorMaterializer()
+  implicit val patience = PatienceConfig(1.second)
 
   def graph(async: Boolean) =
     Source.unfoldInf(1)(x ⇒ (x, x)).filter(_ % 2 == 1)
@@ -87,4 +92,38 @@ class FusingSpec extends AkkaSpec with ConversionCheckedTripleEquals {
 
   }
 
+  "SubFusingActorMaterializer" must {
+
+    "work with asynchronous boundaries in the subflows" in {
+      val async = Flow[Int].map(_ * 2).withAttributes(Attributes.asyncBoundary)
+      Source(0 to 9)
+        .map(_ * 10)
+        .flatMapMerge(5, i ⇒ Source(i to (i + 9)).via(async))
+        .grouped(1000)
+        .runWith(Sink.head)
+        .futureValue
+        .sorted should ===(0 to 198 by 2)
+    }
+
+    "use multiple actors when there are asynchronous boundaries in the subflows" in {
+      def ref = {
+        val bus = GraphInterpreter.currentInterpreter.log.asInstanceOf[BusLogging]
+        bus.logSource
+      }
+      val async = Flow[Int].map(x ⇒ { testActor ! ref; x }).withAttributes(Attributes.asyncBoundary)
+      Source(0 to 9)
+        .map(x ⇒ { testActor ! ref; x })
+        .flatMapMerge(5, i ⇒ Source.single(i).via(async))
+        .grouped(1000)
+        .runWith(Sink.head)
+        .futureValue
+        .sorted should ===(0 to 9)
+      val refs = receiveN(20)
+      withClue(s"refs=\n${refs.mkString("\n")}") {
+        refs.toSet.size should ===(11)
+      }
+    }
+
+  }
+
 }
diff --git a/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterSpec.scala b/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterSpec.scala
index 61870f2e95..06a0060251 100644
--- a/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterSpec.scala
+++ b/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterSpec.scala
@@ -56,7 +56,7 @@ class GraphInterpreterSpec extends AkkaSpec with GraphInterpreterSpecKit {
       manualInit(assembly)
       interpreter.attachDownstreamBoundary(2, sink)
       interpreter.attachUpstreamBoundary(0, source)
-      interpreter.init()
+      interpreter.init(null)
 
       lastEvents() should ===(Set.empty)
 
diff --git a/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterSpecKit.scala b/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterSpecKit.scala
index a66dd6f5bd..d408e5dc96 100644
--- a/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterSpecKit.scala
+++ b/akka-stream-tests/src/test/scala/akka/stream/impl/fusing/GraphInterpreterSpecKit.scala
@@ -84,7 +84,7 @@ trait GraphInterpreterSpecKit extends AkkaSpec {
           _interpreter.attachDownstreamBoundary(i + upstreams.size + connections.size, downstream._2)
         }
 
-        _interpreter.init()
+        _interpreter.init(null)
       }
     }
 
@@ -226,7 +226,7 @@ trait GraphInterpreterSpecKit extends AkkaSpec {
     manualInit(assembly)
     interpreter.attachDownstreamBoundary(0, in)
     interpreter.attachUpstreamBoundary(0, out)
-    interpreter.init()
+    interpreter.init(null)
   }
 
   abstract class FailingStageSetup(initFailOnNextEvent: Boolean = false) extends TestSetup {
@@ -351,7 +351,7 @@ trait GraphInterpreterSpecKit extends AkkaSpec {
       interpreter.attachUpstreamBoundary(0, upstream)
       interpreter.attachDownstreamBoundary(ops.length, downstream)
 
-      interpreter.init()
+      interpreter.init(null)
 
     }
 
diff --git a/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowSpec.scala b/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowSpec.scala
index 43a049d01e..416bdc2a8d 100644
--- a/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowSpec.scala
+++ b/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowSpec.scala
@@ -22,6 +22,7 @@ import scala.collection.immutable
 import scala.concurrent.Await
 import scala.concurrent.duration._
 import scala.util.control.NoStackTrace
+import akka.stream.impl.fusing.GraphInterpreterShell
 
 object FlowSpec {
   class Fruit
@@ -41,22 +42,14 @@ class FlowSpec extends AkkaSpec(ConfigFactory.parseString("akka.actor.debug.rece
   val identity: Flow[Any, Any, Unit] ⇒ Flow[Any, Any, Unit] = in ⇒ in.map(e ⇒ e)
   val identity2: Flow[Any, Any, Unit] ⇒ Flow[Any, Any, Unit] = in ⇒ identity(in)
 
-  class BrokenActorInterpreter(
-    _assembly: GraphAssembly,
-    _inHandlers: Array[InHandler],
-    _outHandlers: Array[OutHandler],
-    _logics: Array[GraphStageLogic],
-    _shape: Shape,
-    _settings: ActorMaterializerSettings,
-    _materializer: Materializer,
-    brokenMessage: Any)
-    extends ActorGraphInterpreter(_assembly, _inHandlers, _outHandlers, _logics, _shape, _settings, _materializer) {
+  class BrokenActorInterpreter(_shell: GraphInterpreterShell, brokenMessage: Any)
+    extends ActorGraphInterpreter(_shell) {
 
     import akka.stream.actor.ActorSubscriberMessage._
 
     override protected[akka] def aroundReceive(receive: Receive, msg: Any) = {
       msg match {
-        case ActorGraphInterpreter.OnNext(0, m) if m == brokenMessage ⇒
+        case ActorGraphInterpreter.OnNext(_, 0, m) if m == brokenMessage ⇒
           throw new NullPointerException(s"I'm so broken [$m]")
         case _ ⇒ super.aroundReceive(receive, msg)
       }
@@ -77,14 +70,17 @@ class FlowSpec extends AkkaSpec(ConfigFactory.parseString("akka.actor.debug.rece
     val (inHandlers, outHandlers, logics) =
       assembly.materialize(Attributes.none, assembly.stages.map(_.module), new java.util.HashMap, _ ⇒ ())
 
-    val props = Props(new BrokenActorInterpreter(assembly, inHandlers, outHandlers, logics, stage.shape, settings, materializer, "a3"))
+    val shell = new GraphInterpreterShell(assembly, inHandlers, outHandlers, logics, stage.shape, settings,
+      materializer.asInstanceOf[ActorMaterializerImpl])
+
+    val props = Props(new BrokenActorInterpreter(shell, "a3"))
       .withDispatcher("akka.test.stream-dispatcher").withDeploy(Deploy.local)
     val impl = system.actorOf(props, "borken-stage-actor")
 
-    val subscriber = new ActorGraphInterpreter.BoundarySubscriber(impl, 0)
-    val publisher = new ActorPublisher[Any](impl) { override val wakeUpMsg = ActorGraphInterpreter.SubscribePending(0) }
+    val subscriber = new ActorGraphInterpreter.BoundarySubscriber(impl, shell, 0)
+    val publisher = new ActorPublisher[Any](impl) { override val wakeUpMsg = ActorGraphInterpreter.SubscribePending(shell, 0) }
 
-    impl ! ActorGraphInterpreter.ExposedPublisher(0, publisher)
+    impl ! ActorGraphInterpreter.ExposedPublisher(shell, 0, publisher)
 
     Flow.fromSinkAndSource(Sink(subscriber), Source(publisher))
   })
diff --git a/akka-stream/src/main/scala/akka/stream/impl/ActorMaterializerImpl.scala b/akka-stream/src/main/scala/akka/stream/impl/ActorMaterializerImpl.scala
index dee5691de6..4e946fe6d6 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/ActorMaterializerImpl.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/ActorMaterializerImpl.scala
@@ -20,6 +20,7 @@ import scala.concurrent.{ Await, ExecutionContextExecutor }
 import akka.stream.impl.fusing.GraphStageModule
 import akka.stream.impl.fusing.GraphInterpreter.GraphAssembly
 import akka.stream.impl.fusing.Fusing
+import akka.stream.impl.fusing.GraphInterpreterShell
 
 /**
  * INTERNAL API
@@ -44,7 +45,7 @@ private[akka] case class ActorMaterializerImpl(system: ActorSystem,
 
   override def isShutdown: Boolean = haveShutDown.get()
 
-  override def withNamePrefix(name: String): Materializer = this.copy(flowNames = flowNames.copy(name))
+  override def withNamePrefix(name: String): ActorMaterializerImpl = this.copy(flowNames = flowNames.copy(name))
 
   private[this] def createFlowName(): String = flowNames.next()
 
@@ -73,7 +74,11 @@ private[akka] case class ActorMaterializerImpl(system: ActorSystem,
   override def scheduleOnce(delay: FiniteDuration, task: Runnable) =
     system.scheduler.scheduleOnce(delay, task)(executionContext)
 
-  override def materialize[Mat](_runnableGraph: Graph[ClosedShape, Mat]): Mat = {
+  override def materialize[Mat](_runnableGraph: Graph[ClosedShape, Mat]): Mat =
+    materialize(_runnableGraph, null)
+
+  private[stream] def materialize[Mat](_runnableGraph: Graph[ClosedShape, Mat],
+                                       subflowFuser: GraphInterpreterShell ⇒ ActorRef): Mat = {
     val runnableGraph =
       if (settings.autoFusing) Fusing.aggressive(_runnableGraph)
       else _runnableGraph
@@ -146,17 +151,24 @@ private[akka] case class ActorMaterializerImpl(system: ActorSystem,
         val calculatedSettings = effectiveSettings(effectiveAttributes)
         val (inHandlers, outHandlers, logics) = graph.assembly.materialize(effectiveAttributes, graph.matValIDs, matVal, registerSrc)
 
-        val props = ActorGraphInterpreter.props(
-          graph.assembly, inHandlers, outHandlers, logics, graph.shape, calculatedSettings, ActorMaterializerImpl.this)
+        val shell = new GraphInterpreterShell(graph.assembly, inHandlers, outHandlers, logics, graph.shape,
+          calculatedSettings, ActorMaterializerImpl.this)
+
+        val impl =
+          if (subflowFuser != null && !effectiveAttributes.contains(Attributes.AsyncBoundary)) {
+            subflowFuser(shell)
+          } else {
+            val props = ActorGraphInterpreter.props(shell)
+            actorOf(props, stageName(effectiveAttributes), calculatedSettings.dispatcher)
+          }
 
-        val impl = actorOf(props, stageName(effectiveAttributes), calculatedSettings.dispatcher)
         for ((inlet, i) ← graph.shape.inlets.iterator.zipWithIndex) {
-          val subscriber = new ActorGraphInterpreter.BoundarySubscriber(impl, i)
+          val subscriber = new ActorGraphInterpreter.BoundarySubscriber(impl, shell, i)
           assignPort(inlet, subscriber)
         }
         for ((outlet, i) ← graph.shape.outlets.iterator.zipWithIndex) {
-          val publisher = new ActorPublisher[Any](impl) { override val wakeUpMsg = ActorGraphInterpreter.SubscribePending(i) }
-          impl ! ActorGraphInterpreter.ExposedPublisher(i, publisher)
+          val publisher = new ActorGraphInterpreter.BoundaryPublisher(impl, shell, i)
+          impl ! ActorGraphInterpreter.ExposedPublisher(shell, i, publisher)
           assignPort(outlet, publisher)
         }
       }
@@ -207,6 +219,20 @@ private[akka] case class ActorMaterializerImpl(system: ActorSystem,
 
 }
 
+private[akka] class SubFusingActorMaterializerImpl(val delegate: ActorMaterializerImpl, registerShell: GraphInterpreterShell ⇒ ActorRef) extends Materializer {
+  override def executionContext: ExecutionContextExecutor = delegate.executionContext
+
+  override def materialize[Mat](runnable: Graph[ClosedShape, Mat]): Mat = delegate.materialize(runnable, registerShell)
+
+  override def scheduleOnce(delay: FiniteDuration, task: Runnable): Cancellable = delegate.scheduleOnce(delay, task)
+
+  override def schedulePeriodically(initialDelay: FiniteDuration, interval: FiniteDuration, task: Runnable): Cancellable =
+    delegate.schedulePeriodically(initialDelay, interval, task)
+
+  def withNamePrefix(name: String): SubFusingActorMaterializerImpl =
+    new SubFusingActorMaterializerImpl(delegate.withNamePrefix(name), registerShell)
+}
+
 /**
  * INTERNAL API
  */
diff --git a/akka-stream/src/main/scala/akka/stream/impl/fusing/ActorGraphInterpreter.scala b/akka-stream/src/main/scala/akka/stream/impl/fusing/ActorGraphInterpreter.scala
index 7de5f225ee..6267d26e2b 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/fusing/ActorGraphInterpreter.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/fusing/ActorGraphInterpreter.scala
@@ -4,7 +4,6 @@
 package akka.stream.impl.fusing
 
 import java.util.concurrent.TimeoutException
-
 import akka.actor._
 import akka.event.Logging
 import akka.stream._
@@ -14,9 +13,11 @@ import akka.stream.impl.fusing.GraphInterpreter.{ DownstreamBoundaryStageLogic,
 import akka.stream.impl.{ ActorPublisher, ReactiveStreamsCompliance }
 import akka.stream.stage.{ GraphStageLogic, InHandler, OutHandler }
 import org.reactivestreams.{ Subscriber, Subscription }
-
 import scala.concurrent.forkjoin.ThreadLocalRandom
 import scala.util.control.NonFatal
+import akka.event.LoggingAdapter
+import akka.stream.impl.ActorMaterializerImpl
+import akka.stream.impl.SubFusingActorMaterializerImpl
 
 /**
  * INTERNAL API
@@ -41,52 +42,53 @@ private[stream] case class GraphModule(assembly: GraphAssembly, shape: Shape, at
  * INTERNAL API
  */
 private[stream] object ActorGraphInterpreter {
-  trait BoundaryEvent extends DeadLetterSuppression with NoSerializationVerificationNeeded
+  trait BoundaryEvent extends DeadLetterSuppression with NoSerializationVerificationNeeded {
+    def shell: GraphInterpreterShell
+  }
 
-  final case class OnError(id: Int, cause: Throwable) extends BoundaryEvent
-  final case class OnComplete(id: Int) extends BoundaryEvent
-  final case class OnNext(id: Int, e: Any) extends BoundaryEvent
-  final case class OnSubscribe(id: Int, subscription: Subscription) extends BoundaryEvent
+  final case class OnError(shell: GraphInterpreterShell, id: Int, cause: Throwable) extends BoundaryEvent
+  final case class OnComplete(shell: GraphInterpreterShell, id: Int) extends BoundaryEvent
+  final case class OnNext(shell: GraphInterpreterShell, id: Int, e: Any) extends BoundaryEvent
+  final case class OnSubscribe(shell: GraphInterpreterShell, id: Int, subscription: Subscription) extends BoundaryEvent
 
-  final case class RequestMore(id: Int, demand: Long) extends BoundaryEvent
-  final case class Cancel(id: Int) extends BoundaryEvent
-  final case class SubscribePending(id: Int) extends BoundaryEvent
-  final case class ExposedPublisher(id: Int, publisher: ActorPublisher[Any]) extends BoundaryEvent
+  final case class RequestMore(shell: GraphInterpreterShell, id: Int, demand: Long) extends BoundaryEvent
+  final case class Cancel(shell: GraphInterpreterShell, id: Int) extends BoundaryEvent
+  final case class SubscribePending(shell: GraphInterpreterShell, id: Int) extends BoundaryEvent
+  final case class ExposedPublisher(shell: GraphInterpreterShell, id: Int, publisher: ActorPublisher[Any]) extends BoundaryEvent
 
-  final case class AsyncInput(logic: GraphStageLogic, evt: Any, handler: (Any) ⇒ Unit) extends BoundaryEvent
+  final case class AsyncInput(shell: GraphInterpreterShell, logic: GraphStageLogic, evt: Any, handler: (Any) ⇒ Unit) extends BoundaryEvent
 
-  case object Resume extends BoundaryEvent
+  case class Resume(shell: GraphInterpreterShell) extends BoundaryEvent
+  case class Abort(shell: GraphInterpreterShell) extends BoundaryEvent
 
-  final class BoundarySubscription(val parent: ActorRef, val id: Int) extends Subscription {
-    override def request(elements: Long): Unit = parent ! RequestMore(id, elements)
-    override def cancel(): Unit = parent ! Cancel(id)
+  final class BoundaryPublisher(parent: ActorRef, shell: GraphInterpreterShell, id: Int) extends ActorPublisher[Any](parent) {
+    override val wakeUpMsg = ActorGraphInterpreter.SubscribePending(shell, id)
+  }
+
+  final class BoundarySubscription(parent: ActorRef, shell: GraphInterpreterShell, id: Int) extends Subscription {
+    override def request(elements: Long): Unit = parent ! RequestMore(shell, id, elements)
+    override def cancel(): Unit = parent ! Cancel(shell, id)
     override def toString = s"BoundarySubscription[$parent, $id]"
   }
 
-  final class BoundarySubscriber(val parent: ActorRef, id: Int) extends Subscriber[Any] {
+  final class BoundarySubscriber(parent: ActorRef, shell: GraphInterpreterShell, id: Int) extends Subscriber[Any] {
     override def onError(cause: Throwable): Unit = {
       ReactiveStreamsCompliance.requireNonNullException(cause)
-      parent ! OnError(id, cause)
+      parent ! OnError(shell, id, cause)
     }
-    override def onComplete(): Unit = parent ! OnComplete(id)
+    override def onComplete(): Unit = parent ! OnComplete(shell, id)
     override def onNext(element: Any): Unit = {
       ReactiveStreamsCompliance.requireNonNullElement(element)
-      parent ! OnNext(id, element)
+      parent ! OnNext(shell, id, element)
     }
     override def onSubscribe(subscription: Subscription): Unit = {
       ReactiveStreamsCompliance.requireNonNullSubscription(subscription)
-      parent ! OnSubscribe(id, subscription)
+      parent ! OnSubscribe(shell, id, subscription)
     }
   }
 
-  def props(assembly: GraphAssembly,
-            inHandlers: Array[InHandler],
-            outHandlers: Array[OutHandler],
-            logics: Array[GraphStageLogic],
-            shape: Shape,
-            settings: ActorMaterializerSettings,
-            mat: Materializer): Props =
-    Props(new ActorGraphInterpreter(assembly, inHandlers, outHandlers, logics, shape, settings, mat)).withDeploy(Deploy.local)
+  def props(shell: GraphInterpreterShell): Props =
+    Props(new ActorGraphInterpreter(shell)).withDeploy(Deploy.local)
 
   class BatchingActorInputBoundary(size: Int, id: Int) extends UpstreamBoundaryStageLogic[Any] {
     require(size > 0, "buffer size cannot be zero")
@@ -201,7 +203,7 @@ private[stream] object ActorGraphInterpreter {
     override def toString: String = s"BatchingActorInputBoundary(id=$id, fill=$inputBufferElements/$size, completed=$upstreamCompleted, canceled=$downstreamCanceled)"
   }
 
-  private[stream] class ActorOutputBoundary(actor: ActorRef, id: Int) extends DownstreamBoundaryStageLogic[Any] {
+  private[stream] class ActorOutputBoundary(actor: ActorRef, shell: GraphInterpreterShell, id: Int) extends DownstreamBoundaryStageLogic[Any] {
     val in: Inlet[Any] = Inlet[Any]("UpstreamBoundary" + id)
     in.id = 0
 
@@ -258,7 +260,7 @@ private[stream] object ActorGraphInterpreter {
       exposedPublisher.takePendingSubscribers() foreach { sub ⇒
         if (subscriber eq null) {
           subscriber = sub
-          tryOnSubscribe(subscriber, new BoundarySubscription(actor, id))
+          tryOnSubscribe(subscriber, new BoundarySubscription(actor, shell, id))
           if (GraphInterpreter.Debug) println(s"${interpreter.Name}  subscribe subscriber=$sub")
         } else
           rejectAdditionalSubscriber(subscriber, s"${Logging.simpleName(this)}")
@@ -301,28 +303,25 @@ private[stream] object ActorGraphInterpreter {
 /**
  * INTERNAL API
  */
-private[stream] class ActorGraphInterpreter(
+private[stream] final class GraphInterpreterShell(
   assembly: GraphAssembly,
   inHandlers: Array[InHandler],
   outHandlers: Array[OutHandler],
   logics: Array[GraphStageLogic],
   shape: Shape,
   settings: ActorMaterializerSettings,
-  mat: Materializer) extends Actor {
+  mat: ActorMaterializerImpl) {
+
   import ActorGraphInterpreter._
 
-  val interpreter = new GraphInterpreter(
-    assembly,
-    mat,
-    Logging(this),
-    inHandlers,
-    outHandlers,
-    logics,
-    (logic, event, handler) ⇒ self ! AsyncInput(logic, event, handler),
-    settings.fuzzingMode)
+  private var self: ActorRef = _
+  lazy val log = Logging(mat.system.eventStream, self)
 
-  private val inputs = Array.tabulate(shape.inlets.size)(new BatchingActorInputBoundary(settings.maxInputBufferSize, _))
-  private val outputs = Array.tabulate(shape.outlets.size)(new ActorOutputBoundary(self, _))
+  lazy val interpreter = new GraphInterpreter(assembly, mat, log, inHandlers, outHandlers, logics,
+    (logic, event, handler) ⇒ self ! AsyncInput(this, logic, event, handler), settings.fuzzingMode)
+
+  private val inputs = new Array[BatchingActorInputBoundary](shape.inlets.size)
+  private val outputs = new Array[ActorOutputBoundary](shape.outlets.size)
 
   private var subscribesPending = inputs.length
   private var publishersPending = outputs.length
@@ -333,97 +332,108 @@ private[stream] class ActorGraphInterpreter(
    * to give each input buffer slot a chance to run through the whole pipeline
    * and back (for the demand).
    */
-  val eventLimit = settings.maxInputBufferSize * (assembly.ins.length + assembly.outs.length)
+  private val eventLimit = settings.maxInputBufferSize * (assembly.ins.length + assembly.outs.length)
 
   // Limits the number of events processed by the interpreter on an abort event.
   // TODO: Better heuristic here
   private val abortLimit = eventLimit * 2
   private var resumeScheduled = false
 
-  override def preStart(): Unit = {
+  def init(self: ActorRef, registerShell: GraphInterpreterShell ⇒ ActorRef): Unit = {
+    this.self = self
     var i = 0
     while (i < inputs.length) {
-      interpreter.attachUpstreamBoundary(i, inputs(i))
+      val in = new BatchingActorInputBoundary(settings.maxInputBufferSize, i)
+      inputs(i) = in
+      interpreter.attachUpstreamBoundary(i, in)
       i += 1
     }
     val offset = assembly.connectionCount - outputs.length
     i = 0
     while (i < outputs.length) {
-      interpreter.attachDownstreamBoundary(i + offset, outputs(i))
+      val out = new ActorOutputBoundary(self, this, i)
+      outputs(i) = out
+      interpreter.attachDownstreamBoundary(i + offset, out)
       i += 1
     }
-    interpreter.init()
+    interpreter.init(new SubFusingActorMaterializerImpl(mat, registerShell))
     runBatch()
   }
 
-  override def receive: Receive = {
-    // Cases that are most likely on the hot path, in decreasing order of frequency
-    case OnNext(id: Int, e: Any) ⇒
-      if (GraphInterpreter.Debug) println(s"${interpreter.Name}  onNext $e id=$id")
-      inputs(id).onNext(e)
-      runBatch()
-    case RequestMore(id: Int, demand: Long) ⇒
-      if (GraphInterpreter.Debug) println(s"${interpreter.Name}  request  $demand id=$id")
-      outputs(id).requestMore(demand)
-      runBatch()
-    case Resume ⇒
-      if (GraphInterpreter.Debug) println(s"${interpreter.Name}  resume")
-      resumeScheduled = false
-      if (interpreter.isSuspended) runBatch()
-    case AsyncInput(logic, event, handler) ⇒
-      if (GraphInterpreter.Debug) println(s"${interpreter.Name} ASYNC $event ($handler) [$logic]")
-      if (!interpreter.isStageCompleted(logic)) {
-        try handler(event)
-        catch {
-          case NonFatal(e) ⇒ logic.failStage(e)
+  def receive(event: BoundaryEvent): Unit =
+    if (waitingForShutdown) event match {
+      case ExposedPublisher(_, id, publisher) ⇒
+        outputs(id).exposedPublisher(publisher)
+        publishersPending -= 1
+        if (canShutDown) _isTerminated = true
+      case OnSubscribe(_, _, sub) ⇒
+        tryCancel(sub)
+        subscribesPending -= 1
+        if (canShutDown) _isTerminated = true
+      case Abort(_) ⇒
+        tryAbort(new TimeoutException("Streaming actor has been already stopped processing (normally), but not all of its " +
+          s"inputs or outputs have been subscribed in [${settings.subscriptionTimeoutSettings.timeout}}]. Aborting actor now."))
+      case _ ⇒ // Ignore, there is nothing to do anyway
+    }
+    else event match {
+      // Cases that are most likely on the hot path, in decreasing order of frequency
+      case OnNext(_, id: Int, e: Any) ⇒
+        if (GraphInterpreter.Debug) println(s"${interpreter.Name}  onNext $e id=$id")
+        inputs(id).onNext(e)
+        runBatch()
+      case RequestMore(_, id: Int, demand: Long) ⇒
+        if (GraphInterpreter.Debug) println(s"${interpreter.Name}  request  $demand id=$id")
+        outputs(id).requestMore(demand)
+        runBatch()
+      case Resume(_) ⇒
+        if (GraphInterpreter.Debug) println(s"${interpreter.Name}  resume")
+        resumeScheduled = false
+        if (interpreter.isSuspended) runBatch()
+      case AsyncInput(_, logic, event, handler) ⇒
+        if (GraphInterpreter.Debug) println(s"${interpreter.Name} ASYNC $event ($handler) [$logic]")
+        if (!interpreter.isStageCompleted(logic)) {
+          try handler(event)
+          catch {
+            case NonFatal(e) ⇒ logic.failStage(e)
+          }
+          interpreter.afterStageHasRun(logic)
         }
-        interpreter.afterStageHasRun(logic)
-      }
-      runBatch()
+        runBatch()
 
-    // Initialization and completion messages
-    case OnError(id: Int, cause: Throwable) ⇒
-      if (GraphInterpreter.Debug) println(s"${interpreter.Name}  onError id=$id")
-      inputs(id).onError(cause)
-      runBatch()
-    case OnComplete(id: Int) ⇒
-      if (GraphInterpreter.Debug) println(s"${interpreter.Name}  onComplete id=$id")
-      inputs(id).onComplete()
-      runBatch()
-    case OnSubscribe(id: Int, subscription: Subscription) ⇒
-      if (GraphInterpreter.Debug) println(s"${interpreter.Name}  onSubscribe id=$id")
-      subscribesPending -= 1
-      inputs(id).onSubscribe(subscription)
-      runBatch()
-    case Cancel(id: Int) ⇒
-      if (GraphInterpreter.Debug) println(s"${interpreter.Name}  cancel id=$id")
-      outputs(id).cancel()
-      runBatch()
-    case SubscribePending(id: Int) ⇒
-      outputs(id).subscribePending()
-    case ExposedPublisher(id, publisher) ⇒
-      publishersPending -= 1
-      outputs(id).exposedPublisher(publisher)
+      // Initialization and completion messages
+      case OnError(_, id: Int, cause: Throwable) ⇒
+        if (GraphInterpreter.Debug) println(s"${interpreter.Name}  onError id=$id")
+        inputs(id).onError(cause)
+        runBatch()
+      case OnComplete(_, id: Int) ⇒
+        if (GraphInterpreter.Debug) println(s"${interpreter.Name}  onComplete id=$id")
+        inputs(id).onComplete()
+        runBatch()
+      case OnSubscribe(_, id: Int, subscription: Subscription) ⇒
+        if (GraphInterpreter.Debug) println(s"${interpreter.Name}  onSubscribe id=$id")
+        subscribesPending -= 1
+        inputs(id).onSubscribe(subscription)
+        runBatch()
+      case Cancel(_, id: Int) ⇒
+        if (GraphInterpreter.Debug) println(s"${interpreter.Name}  cancel id=$id")
+        outputs(id).cancel()
+        runBatch()
+      case SubscribePending(_, id: Int) ⇒
+        outputs(id).subscribePending()
+      case ExposedPublisher(_, id, publisher) ⇒
+        publishersPending -= 1
+        outputs(id).exposedPublisher(publisher)
+    }
 
-  }
-
-  private def waitShutdown: Receive = {
-    case ExposedPublisher(id, publisher) ⇒
-      outputs(id).exposedPublisher(publisher)
-      publishersPending -= 1
-      if (canShutDown) context.stop(self)
-    case OnSubscribe(_, sub) ⇒
-      tryCancel(sub)
-      subscribesPending -= 1
-      if (canShutDown) context.stop(self)
-    case ReceiveTimeout ⇒
-      tryAbort(new TimeoutException("Streaming actor has been already stopped processing (normally), but not all of its " +
-        s"inputs or outputs have been subscribed in [${settings.subscriptionTimeoutSettings.timeout}}]. Aborting actor now."))
-    case _ ⇒ // Ignore, there is nothing to do anyway
-  }
+  private var _isTerminated = false
+  def isTerminated: Boolean = _isTerminated
 
   private def canShutDown: Boolean = subscribesPending + publishersPending == 0
 
+  private var waitingForShutdown: Boolean = false
+
+  private val resume = Resume(this)
+
   private def runBatch(): Unit = {
     try {
       val effectiveLimit = {
@@ -436,18 +446,19 @@ private[stream] class ActorGraphInterpreter(
       interpreter.execute(effectiveLimit)
       if (interpreter.isCompleted) {
         // Cannot stop right away if not completely subscribed
-        if (canShutDown) context.stop(self)
+        if (canShutDown) _isTerminated = true
         else {
-          context.become(waitShutdown)
-          context.setReceiveTimeout(settings.subscriptionTimeoutSettings.timeout)
+          waitingForShutdown = true
+          mat.scheduleOnce(settings.subscriptionTimeoutSettings.timeout, new Runnable {
+            override def run(): Unit = self ! Abort(GraphInterpreterShell.this)
+          })
         }
       } else if (interpreter.isSuspended && !resumeScheduled) {
         resumeScheduled = true
-        self ! Resume
+        self ! resume
       }
     } catch {
       case NonFatal(e) ⇒
-        context.stop(self)
         tryAbort(e)
     }
   }
@@ -458,20 +469,57 @@ private[stream] class ActorGraphInterpreter(
    *  - the event limit is reached
    *  - a new error is encountered
    */
-  private def tryAbort(ex: Throwable): Unit = {
+  def tryAbort(ex: Throwable): Unit = {
     // This should handle termination while interpreter is running. If the upstream have been closed already this
     // call has no effect and therefore do the right thing: nothing.
     try {
       inputs.foreach(_.onInternalError(ex))
       interpreter.execute(abortLimit)
       interpreter.finish()
-    } // Will only have an effect if the above call to the interpreter failed to emit a proper failure to the downstream
-    // otherwise this will have no effect
-    finally {
+    } catch {
+      case NonFatal(_) ⇒
+    } finally {
+      _isTerminated = true
+      // Will only have an effect if the above call to the interpreter failed to emit a proper failure to the downstream
+      // otherwise this will have no effect
       outputs.foreach(_.fail(ex))
       inputs.foreach(_.cancel())
     }
   }
 
-  override def postStop(): Unit = tryAbort(AbruptTerminationException(self))
+  override def toString: String = s"GraphInterpreterShell\n  ${assembly.toString.replace("\n", "\n  ")}"
+}
+
+/**
+ * INTERNAL API
+ */
+private[stream] class ActorGraphInterpreter(_initial: GraphInterpreterShell) extends Actor {
+  import ActorGraphInterpreter._
+
+  var activeInterpreters = Set(_initial)
+
+  def registerShell(shell: GraphInterpreterShell): ActorRef = {
+    shell.init(self, registerShell)
+    if (GraphInterpreter.Debug) println(s"registering new shell in ${_initial}\n  ${shell.toString.replace("\n", "\n  ")}")
+    activeInterpreters += shell
+    self
+  }
+
+  override def preStart(): Unit = {
+    activeInterpreters.foreach(_.init(self, registerShell))
+  }
+
+  override def receive: Receive = {
+    case b: BoundaryEvent ⇒
+      val shell = b.shell
+      if (GraphInterpreter.Debug)
+        if (!activeInterpreters.contains(shell)) println(s"RECEIVED EVENT $b FOR UNKNOWN SHELL $shell")
+      shell.receive(b)
+      if (shell.isTerminated) {
+        activeInterpreters -= shell
+        if (activeInterpreters.isEmpty) context.stop(self)
+      }
+  }
+
+  override def postStop(): Unit = activeInterpreters.foreach(_.tryAbort(AbruptTerminationException(self)))
 }
diff --git a/akka-stream/src/main/scala/akka/stream/impl/fusing/GraphInterpreter.scala b/akka-stream/src/main/scala/akka/stream/impl/fusing/GraphInterpreter.scala
index 09db604597..3c653a2341 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/fusing/GraphInterpreter.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/fusing/GraphInterpreter.scala
@@ -372,6 +372,9 @@ private[stream] final class GraphInterpreter(
     shape.inlets.size + shape.outlets.size + keepGoing
   }
 
+  private var _subFusingMaterializer: Materializer = _
+  def subFusingMaterializer: Materializer = _subFusingMaterializer
+
   // An event queue implemented as a circular buffer
   // FIXME: This calculates the maximum size ever needed, but most assemblies can run on a smaller queue
   private[this] val eventQueue = Array.ofDim[Int](1 << (32 - Integer.numberOfLeadingZeros(assembly.connectionCount - 1)))
@@ -445,9 +448,14 @@ private[stream] final class GraphInterpreter(
   def isCompleted: Boolean = runningStages == 0 && !isSuspended
 
   /**
-   * Initializes the states of all the stage logics by calling preStart()
+   * Initializes the states of all the stage logics by calling preStart().
+   * The passed-in materializer is intended to be a SubFusingActorMaterializer
+   * that avoids creating new Actors when stages materialize sub-flows. If no
+   * such materializer is available, passing in `null` will reuse the normal
+   * materializer for the GraphInterpreter—fusing is only an optimization.
    */
-  def init(): Unit = {
+  def init(subMat: Materializer): Unit = {
+    _subFusingMaterializer = if (subMat == null) materializer else subMat
     var i = 0
     while (i < logics.length) {
       val logic = logics(i)
diff --git a/akka-stream/src/main/scala/akka/stream/impl/fusing/IteratorInterpreter.scala b/akka-stream/src/main/scala/akka/stream/impl/fusing/IteratorInterpreter.scala
index fa94d17dd7..0143445314 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/fusing/IteratorInterpreter.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/fusing/IteratorInterpreter.scala
@@ -150,7 +150,7 @@ private[akka] class IteratorInterpreter[I, O](val input: Iterator[I], val ops: S
       fuzzingMode = false)
     interpreter.attachUpstreamBoundary(0, upstream)
     interpreter.attachDownstreamBoundary(ops.length, downstream)
-    interpreter.init()
+    interpreter.init(null)
   }
 
   init()
diff --git a/akka-stream/src/main/scala/akka/stream/impl/fusing/StreamOfStreams.scala b/akka-stream/src/main/scala/akka/stream/impl/fusing/StreamOfStreams.scala
index bf03be2c71..e5783b899e 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/fusing/StreamOfStreams.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/fusing/StreamOfStreams.scala
@@ -125,7 +125,7 @@ final class FlattenMerge[T, M](breadth: Int) extends GraphStage[FlowShape[Graph[
             if (localSource.elem == null) removeSource(localSource)
           case OnError(ex) ⇒
             failStage(ex)
-        }.invoke))(interpreter.materializer)
+        }.invoke))(interpreter.subFusingMaterializer)
       localSource.activate(subF)
     }
 
@@ -140,6 +140,8 @@ final class FlattenMerge[T, M](breadth: Int) extends GraphStage[FlowShape[Graph[
       sources.foreach(_.cancel())
     }
   }
+
+  override def toString: String = s"FlattenMerge($breadth)"
 }
 
 /**
@@ -285,8 +287,8 @@ object PrefixAndTail {
       override def completeSubstream(): Unit = onParentFinish.invoke(())
       override def failSubstream(ex: Throwable): Unit = onParentFailure.invoke(ex)
 
-      override def onPull(): Unit = pullParent()
-      override def onDownstreamFinish(): Unit = cancelParent()
+      override def onPull(): Unit = pullParent(())
+      override def onDownstreamFinish(): Unit = cancelParent(())
     }
 
     override def createLogic(inheritedAttributes: Attributes): GraphStageLogic = new TailSourceLogic(shape)
@@ -413,4 +415,6 @@ final class PrefixAndTail[T](n: Int) extends GraphStage[FlowShape[T, (immutable.
   }
 
   override def createLogic(inheritedAttributes: Attributes): GraphStageLogic = new PrefixAndTailLogic(shape)
-}
\ No newline at end of file
+
+  override def toString: String = s"PrefixAndTail($n)"
+}
diff --git a/akka-stream/src/main/scala/akka/stream/stage/GraphStage.scala b/akka-stream/src/main/scala/akka/stream/stage/GraphStage.scala
index 2a57396f85..565e3213de 100644
--- a/akka-stream/src/main/scala/akka/stream/stage/GraphStage.scala
+++ b/akka-stream/src/main/scala/akka/stream/stage/GraphStage.scala
@@ -296,6 +296,19 @@ abstract class GraphStageLogic private[stream] (val inCount: Int, val outCount:
       throw new IllegalStateException("not yet initialized: only setHandler is allowed in GraphStageLogic constructor")
     else _interpreter
 
+  /**
+   * The [[akka.stream.Materializer]] that has set this GraphStage in motion.
+   */
+  protected def materializer: Materializer = interpreter.materializer
+
+  /**
+   * An [[akka.stream.Materializer]] that may run fusable parts of the graphs
+   * that it materializes within the same actor as the current GraphStage (if
+   * fusing is available). This materializer must not be shared outside of the
+   * GraphStage.
+   */
+  protected def subFusingMaterializer: Materializer = interpreter.subFusingMaterializer
+
   /**
    * Input handler that terminates the stage upon receiving completion.
    * The stage fails upon receiving a failure.