diff --git a/scripts/fiber/tests-passing.txt b/scripts/fiber/tests-passing.txt
index de017935ce..3bd15a1703 100644
--- a/scripts/fiber/tests-passing.txt
+++ b/scripts/fiber/tests-passing.txt
@@ -751,11 +751,11 @@ src/renderers/dom/fiber/__tests__/ReactDOMFiber-test.js
 
 src/renderers/dom/fiber/__tests__/ReactDOMFiberAsync-test.js
 * renders synchronously by default
-* throws when calling async APIs when feature flag is disabled
-* unstable_asyncUpdates creates an async subtree
-* updates inside an async subtree are async by default
+* renders syncrhonously when feature flag is disabled
 * unstable_asyncUpdates at the root makes the entire tree async
 * updates inside an async tree are async by default
+* unstable_asyncUpdates creates an async subtree
+* updates inside an async subtree are async by default
 
 src/renderers/dom/shared/__tests__/CSSProperty-test.js
 * should generate browser prefixes for its `isUnitlessNumber`
diff --git a/src/renderers/dom/fiber/ReactDOMFiber.js b/src/renderers/dom/fiber/ReactDOMFiber.js
index 7dd9c8a3f2..f4710f49e3 100644
--- a/src/renderers/dom/fiber/ReactDOMFiber.js
+++ b/src/renderers/dom/fiber/ReactDOMFiber.js
@@ -364,7 +364,6 @@ var DOMRenderer = ReactFiberReconciler({
   scheduleDeferredCallback: ReactDOMFrameScheduling.rIC,
 
   useSyncScheduling: !ReactDOMFeatureFlags.fiberAsyncScheduling,
-  enableAsyncSubtreeAPI: ReactDOMFeatureFlags.enableAsyncSubtreeAPI,
 });
 
 ReactGenericBatching.injection.injectFiberBatchedUpdates(
diff --git a/src/renderers/dom/fiber/__tests__/ReactDOMFiberAsync-test.js b/src/renderers/dom/fiber/__tests__/ReactDOMFiberAsync-test.js
index 746de0e830..9f780ac186 100644
--- a/src/renderers/dom/fiber/__tests__/ReactDOMFiberAsync-test.js
+++ b/src/renderers/dom/fiber/__tests__/ReactDOMFiberAsync-test.js
@@ -1,5 +1,6 @@
 var React = require('react');
 var ReactDOMFeatureFlags = require('ReactDOMFeatureFlags');
+var ReactFeatureFlags = require('ReactFeatureFlags');
 
 var ReactDOM;
 
@@ -23,21 +24,69 @@ describe('ReactDOMFiberAsync', () => {
   });
 
   if (ReactDOMFeatureFlags.useFiber) {
-    it('throws when calling async APIs when feature flag is disabled', () => {
-      expect(() => {
-        ReactDOM.unstable_asyncRender(<div>Hi</div>, container);
-      }).toThrow('ReactDOM.unstable_asyncRender is not a function');
+    it('renders syncrhonously when feature flag is disabled', () => {
+      function Async(props) {
+        return props.children;
+      }
+      Async.unstable_asyncUpdates = true;
+      ReactDOM.render(<Async><div>Hi</div></Async>, container);
+      expect(container.textContent).toEqual('Hi');
+
+      ReactDOM.render(<Async><div>Bye</div></Async>, container);
+      expect(container.textContent).toEqual('Bye');
     });
 
     describe('with feature flag enabled', () => {
       beforeEach(() => {
         jest.resetModules();
-        ReactDOMFeatureFlags = require('ReactDOMFeatureFlags');
+        ReactFeatureFlags = require('ReactFeatureFlags');
         container = document.createElement('div');
-        ReactDOMFeatureFlags.enableAsyncSubtreeAPI = true;
+        ReactFeatureFlags.enableAsyncSubtreeAPI = true;
         ReactDOM = require('react-dom');
       });
 
+      it('unstable_asyncUpdates at the root makes the entire tree async', () => {
+        function Async(props) {
+          return props.children;
+        }
+        Async.unstable_asyncUpdates = true;
+        ReactDOM.render(<Async><div>Hi</div></Async>, container);
+        expect(container.textContent).toEqual('');
+        jest.runAllTimers();
+        expect(container.textContent).toEqual('Hi');
+
+        ReactDOM.render(<Async><div>Bye</div></Async>, container);
+        expect(container.textContent).toEqual('Hi');
+        jest.runAllTimers();
+        expect(container.textContent).toEqual('Bye');
+      });
+
+      it('updates inside an async tree are async by default', () => {
+        function Async(props) {
+          return props.children;
+        }
+        Async.unstable_asyncUpdates = true;
+
+        let instance;
+        class Component extends React.Component {
+          state = {step: 0};
+          render() {
+            instance = this;
+            return <div>{this.state.step}</div>;
+          }
+        }
+
+        ReactDOM.render(<Async><Component /></Async>, container);
+        expect(container.textContent).toEqual('');
+        jest.runAllTimers();
+        expect(container.textContent).toEqual('0');
+
+        instance.setState({step: 1});
+        expect(container.textContent).toEqual('0');
+        jest.runAllTimers();
+        expect(container.textContent).toEqual('1');
+      });
+
       it('unstable_asyncUpdates creates an async subtree', () => {
         let instance;
         class Component extends React.Component {
@@ -81,48 +130,6 @@ describe('ReactDOMFiberAsync', () => {
         jest.runAllTimers();
         expect(container.textContent).toEqual('1');
       });
-
-      it('unstable_asyncUpdates at the root makes the entire tree async', () => {
-        function Async(props) {
-          return props.children;
-        }
-        Async.unstable_asyncUpdates = true;
-        ReactDOM.render(<Async><div>Hi</div></Async>, container);
-        expect(container.textContent).toEqual('');
-        jest.runAllTimers();
-        expect(container.textContent).toEqual('Hi');
-
-        ReactDOM.render(<Async><div>Bye</div></Async>, container);
-        expect(container.textContent).toEqual('Hi');
-        jest.runAllTimers();
-        expect(container.textContent).toEqual('Bye');
-      });
-
-      it('updates inside an async tree are async by default', () => {
-        function Async(props) {
-          return props.children;
-        }
-        Async.unstable_asyncUpdates = true;
-
-        let instance;
-        class Component extends React.Component {
-          state = {step: 0};
-          render() {
-            instance = this;
-            return <div>{this.state.step}</div>;
-          }
-        }
-
-        ReactDOM.render(<Async><Component /></Async>, container);
-        expect(container.textContent).toEqual('');
-        jest.runAllTimers();
-        expect(container.textContent).toEqual('0');
-
-        instance.setState({step: 1});
-        expect(container.textContent).toEqual('0');
-        jest.runAllTimers();
-        expect(container.textContent).toEqual('1');
-      });
     });
   }
 });
diff --git a/src/renderers/dom/shared/ReactDOMFeatureFlags.js b/src/renderers/dom/shared/ReactDOMFeatureFlags.js
index 7fb34ceb23..1e21bca758 100644
--- a/src/renderers/dom/shared/ReactDOMFeatureFlags.js
+++ b/src/renderers/dom/shared/ReactDOMFeatureFlags.js
@@ -13,7 +13,6 @@
 
 var ReactDOMFeatureFlags = {
   fiberAsyncScheduling: false,
-  enableAsyncSubtreeAPI: false,
   useCreateElement: true,
   useFiber: true,
 };
diff --git a/src/renderers/shared/fiber/ReactFiber.js b/src/renderers/shared/fiber/ReactFiber.js
index 3eeba7af00..abd9face24 100644
--- a/src/renderers/shared/fiber/ReactFiber.js
+++ b/src/renderers/shared/fiber/ReactFiber.js
@@ -23,6 +23,8 @@ import type {TypeOfSideEffect} from 'ReactTypeOfSideEffect';
 import type {PriorityLevel} from 'ReactPriorityLevel';
 import type {UpdateQueue} from 'ReactFiberUpdateQueue';
 
+var ReactFeatureFlags = require('ReactFeatureFlags');
+
 var {
   IndeterminateComponent,
   ClassComponent,
@@ -374,7 +376,11 @@ function createFiberFromElementType(
   internalContextTag: TypeOfInternalContext,
   debugOwner: null | Fiber | ReactInstance,
 ): Fiber {
-  if (type != null && (type: any).unstable_asyncUpdates === true) {
+  if (
+    ReactFeatureFlags.enableAsyncSubtreeAPI &&
+    type != null &&
+    (type: any).unstable_asyncUpdates === true
+  ) {
     internalContextTag |= AsyncUpdates;
   }
 
diff --git a/src/renderers/shared/fiber/ReactFiberReconciler.js b/src/renderers/shared/fiber/ReactFiberReconciler.js
index 309ede457f..3c95e39c97 100644
--- a/src/renderers/shared/fiber/ReactFiberReconciler.js
+++ b/src/renderers/shared/fiber/ReactFiberReconciler.js
@@ -17,6 +17,8 @@ import type {FiberRoot} from 'ReactFiberRoot';
 import type {PriorityLevel} from 'ReactPriorityLevel';
 import type {ReactNodeList} from 'ReactTypes';
 
+var ReactFeatureFlags = require('ReactFeatureFlags');
+
 var {
   AsyncUpdates,
 } = require('ReactTypeOfInternalContext');
@@ -119,7 +121,6 @@ export type HostConfig<T, P, I, TI, PI, C, CX, PL> = {
   resetAfterCommit(): void,
 
   useSyncScheduling?: boolean,
-  enableAsyncSubtreeAPI?: boolean,
 };
 
 export type Reconciler<C, I, TI> = {
@@ -189,7 +190,8 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     // Check if the top-level element is an async wrapper component. If so, treat
     // updates to the root as async. This is a bit weird but lets us avoid a separate
     // `renderAsync` API.
-    const forceAsync = element != null &&
+    const forceAsync = ReactFeatureFlags.enableAsyncSubtreeAPI &&
+      element != null &&
       element.type != null &&
       (element.type: any).unstable_asyncUpdates === true;
     const priorityLevel = getPriorityContext(current, forceAsync);
diff --git a/src/renderers/shared/fiber/ReactFiberScheduler.js b/src/renderers/shared/fiber/ReactFiberScheduler.js
index c2e05aefd2..fbc6ea62b5 100644
--- a/src/renderers/shared/fiber/ReactFiberScheduler.js
+++ b/src/renderers/shared/fiber/ReactFiberScheduler.js
@@ -169,7 +169,6 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     useSyncScheduling,
     prepareForCommit,
     resetAfterCommit,
-    enableAsyncSubtreeAPI,
   } = config;
 
   // The priority level to use when scheduling an update. We use NoWork to
@@ -1353,8 +1352,7 @@ module.exports = function<T, P, I, TI, PI, C, CX, PL>(
     if (priorityLevel === NoWork) {
       if (
         !useSyncScheduling ||
-        (enableAsyncSubtreeAPI === true &&
-          fiber.internalContextTag & AsyncUpdates) ||
+        fiber.internalContextTag & AsyncUpdates ||
         forceAsync
       ) {
         priorityLevel = LowPriority;
diff --git a/src/renderers/shared/utils/ReactFeatureFlags.js b/src/renderers/shared/utils/ReactFeatureFlags.js
index ce0a442c11..60e91fb64f 100644
--- a/src/renderers/shared/utils/ReactFeatureFlags.js
+++ b/src/renderers/shared/utils/ReactFeatureFlags.js
@@ -19,6 +19,7 @@ var ReactFeatureFlags = {
   logTopLevelRenders: false,
   prepareNewChildrenBeforeUnmountInStack: true,
   disableNewFiberFeatures: false,
+  enableAsyncSubtreeAPI: false,
 };
 
 module.exports = ReactFeatureFlags;