diff --git a/spec/operators/mergeScan-spec.js b/spec/operators/mergeScan-spec.js
new file mode 100644
index 0000000000..9a16bd8b21
--- /dev/null
+++ b/spec/operators/mergeScan-spec.js
@@ -0,0 +1,106 @@
+/* globals describe, it, expect, expectObservable, hot */
+var Rx = require('../../dist/cjs/Rx');
+var Observable = Rx.Observable;
+
+describe('Observable.prototype.mergeScan()', function () {
+  it('should mergeScan things', function () {
+    var e1 = hot('--a--^--b--c--d--e--f--g--|');
+    var expected =    '---u--v--w--x--y--z--|';
+
+    var values = {
+      u: ['b'],
+      v: ['b', 'c'],
+      w: ['b', 'c', 'd'],
+      x: ['b', 'c', 'd', 'e'],
+      y: ['b', 'c', 'd', 'e', 'f'],
+      z: ['b', 'c', 'd', 'e', 'f', 'g']
+    };
+
+    var source = e1.mergeScan(function (acc, x) { return Observable.of(acc.concat(x)); }, []);
+
+    expectObservable(source).toBe(expected, values);
+  });
+
+  it('should handle errors', function () {
+    var e1 = hot('--a--^--b--c--d--#');
+    var expected =    '---u--v--w--#';
+
+    var values = {
+      u: ['b'],
+      v: ['b', 'c'],
+      w: ['b', 'c', 'd']
+    };
+
+    var source = e1.mergeScan(function (acc, x) { return Observable.of(acc.concat(x)); }, []);
+
+    expectObservable(source).toBe(expected, values);
+  });
+
+  it('should handle errors in the projection function', function () {
+    var e1 = hot('--a--^--b--c--d--e--f--g--|');
+    var expected =    '---u--v--#';
+
+    var values = {
+      u: ['b'],
+      v: ['b', 'c']
+    };
+
+    var source = e1.mergeScan(function (acc, x) {
+      if (x === 'd') {
+        throw 'bad!';
+      }
+      return Observable.of(acc.concat(x));
+    }, []);
+
+    expectObservable(source).toBe(expected, values, 'bad!');
+  });
+
+  it('handle empty', function () {
+    var e1 = Observable.empty();
+    var expected = '(u|)';
+
+    var values = {
+        u: []
+    };
+
+    var source = e1.mergeScan(function (acc, x) { return Observable.of(acc.concat(x)); }, []);
+
+    expectObservable(source).toBe(expected, values);
+  });
+
+  it('handle never', function () {
+    var e1 = Observable.never();
+    var expected = '-';
+
+    var source = e1.mergeScan(function (acc, x) { return Observable.of(acc.concat(x)); }, []);
+
+    expectObservable(source).toBe(expected);
+  });
+
+  it('handle throw', function () {
+    var e1 = Observable.throw('bad!');
+    var expected = '#';
+
+    var source = e1.mergeScan(function (acc, x) { return Observable.of(acc.concat(x)); }, []);
+
+    expectObservable(source).toBe(expected, undefined, 'bad!');
+  });
+
+  it('should mergeScan unsubscription', function () {
+    var e1 = hot('--a--^--b--c--d--e--f--g--|');
+    var expected =    '---u--v--w--x--';
+    var sub =         '^             !';
+    var values = {
+      u: ['b'],
+      v: ['b', 'c'],
+      w: ['b', 'c', 'd'],
+      x: ['b', 'c', 'd', 'e'],
+      y: ['b', 'c', 'd', 'e', 'f'],
+      z: ['b', 'c', 'd', 'e', 'f', 'g']
+    };
+
+    var source = e1.mergeScan(function (acc, x) { return Observable.of(acc.concat(x)); }, []);
+
+    expectObservable(source, sub).toBe(expected, values);
+  });
+});
diff --git a/src/Rx.KitchenSink.ts b/src/Rx.KitchenSink.ts
index 773c550538..280b8bb91f 100644
--- a/src/Rx.KitchenSink.ts
+++ b/src/Rx.KitchenSink.ts
@@ -12,6 +12,7 @@ interface KitchenSinkOperators<T> extends CoreOperators<T> {
   max?: <T, R>(comparer?: (x: R, y: T) => R) => Observable<R>;
   min?: <T, R>(comparer?: (x: R, y: T) => R) => Observable<R>;
   timeInterval?: <T>(scheduler?: IScheduler) => Observable<T>;
+  mergeScan?: <T, R>(project: (acc: R, x: T) => Observable<R>, seed: R) => Observable<R>;
 }
 
 // operators
@@ -196,6 +197,9 @@ import mergeMapTo from './operators/mergeMapTo';
 observableProto.mergeMapTo = mergeMapTo;
 observableProto.flatMapTo = mergeMapTo;
 
+import mergeScan from './operators/extended/mergeScan';
+observableProto.mergeScan = mergeScan;
+
 import min from './operators/extended/min';
 observableProto.min = min;
 
diff --git a/src/operators/extended/mergeScan.ts b/src/operators/extended/mergeScan.ts
new file mode 100644
index 0000000000..3118fdcdaa
--- /dev/null
+++ b/src/operators/extended/mergeScan.ts
@@ -0,0 +1,91 @@
+import Operator from '../../Operator';
+import Observable from '../../Observable';
+import Subscriber from '../../Subscriber';
+import Subscription from '../../Subscription';
+import tryCatch from '../../util/tryCatch';
+import { errorObject } from '../../util/errorObject';
+import subscribeToResult from '../../util/subscribeToResult';
+import OuterSubscriber from '../../OuterSubscriber';
+
+export default function mergeScan<T, R>(project: (acc: R, x: T) => Observable<R>, seed: R) {
+  return this.lift(new MergeScanOperator(project, seed));
+}
+
+export class MergeScanOperator<T, R> implements Operator<T, R> {
+  constructor(private project: (acc: R, x: T) => Observable<R>,
+              private seed: R,
+              private concurrent: number = Number.POSITIVE_INFINITY) {
+  }
+
+  call(subscriber: Subscriber<R>): Subscriber<T> {
+    return new MergeScanSubscriber(
+      subscriber, this.project, this.seed, this.concurrent
+    );
+  }
+}
+
+export class MergeScanSubscriber<T, R> extends OuterSubscriber<T, R> {
+  private hasValue: boolean = false;
+  private hasCompleted: boolean = false;
+  private buffer: Observable<any>[] = [];
+  private active: number = 0;
+  protected index: number = 0;
+
+  constructor(destination: Subscriber<R>,
+              private project: (acc: R, x: T) => Observable<R>,
+              private acc: R,
+              private concurrent: number = Number.POSITIVE_INFINITY) {
+    super(destination);
+  }
+
+  _next(value: any): void {
+    if (this.active < this.concurrent) {
+      const index = this.index++;
+      const ish = tryCatch(this.project)(this.acc, value);
+      const destination = this.destination;
+      if (ish === errorObject) {
+        destination.error(ish.e);
+      } else {
+        this.active++;
+        this._innerSub(ish, value, index);
+      }
+    } else {
+      this.buffer.push(value);
+    }
+  }
+
+  _innerSub(ish: any, value: T, index: number): void {
+    this.add(subscribeToResult<T, R>(this, ish, value, index));
+  }
+
+  _complete(): void {
+    this.hasCompleted = true;
+    if (this.active === 0 && this.buffer.length === 0) {
+      if (this.hasValue === false) {
+        this.destination.next(this.acc);
+      }
+      this.destination.complete();
+    }
+  }
+
+  notifyNext(outerValue: T, innerValue: R, outerIndex: number, innerIndex: number): void {
+    const { destination } = this;
+    this.acc = innerValue;
+    this.hasValue = true;
+    destination.next(innerValue);
+  }
+
+  notifyComplete(innerSub: Subscription<T>): void {
+    const buffer = this.buffer;
+    this.remove(innerSub);
+    this.active--;
+    if (buffer.length > 0) {
+      this._next(buffer.shift());
+    } else if (this.active === 0 && this.hasCompleted) {
+      if (this.hasValue === false) {
+        this.destination.next(this.acc);
+      }
+      this.destination.complete();
+    }
+  }
+}