diff --git a/src/operator/mergeAll.ts b/src/operator/mergeAll.ts
index 6d42ef10ae..73fed64cfd 100644
--- a/src/operator/mergeAll.ts
+++ b/src/operator/mergeAll.ts
@@ -3,8 +3,12 @@ import { Operator } from '../Operator';
 import { Observer } from '../Observer';
 import { Subscription } from '../Subscription';
 import { OuterSubscriber } from '../OuterSubscriber';
+import { Subscribable } from '../Observable';
 import { subscribeToResult } from '../util/subscribeToResult';
 
+export function mergeAll<T>(this: Observable<T>, concurrent?: number): T;
+export function mergeAll<T, R>(this: Observable<T>, concurrent?: number): Subscribable<R>;
+
 /**
  * Converts a higher-order Observable into a first-order Observable which
  * concurrently delivers all values that are emitted on the inner Observables.