diff --git a/packages/metro-bundler/src/Bundler/index.js b/packages/metro-bundler/src/Bundler/index.js
index bf33a9fc..6208daa2 100644
--- a/packages/metro-bundler/src/Bundler/index.js
+++ b/packages/metro-bundler/src/Bundler/index.js
@@ -27,6 +27,7 @@ const path = require('path');
 const denodeify = require('denodeify');
 const defaults = require('../defaults');
 const toLocalPath = require('../node-haste/lib/toLocalPath');
+const createModuleIdFactory = require('../lib/createModuleIdFactory');
 
 const {generateAssetTransformResult, isAssetTypeAnImage} = require('./util');
 
@@ -287,10 +288,6 @@ class Bundler {
       );
   }
 
-  getGetModuleIdFn(): ({path: string}) => number {
-    return this._getModuleId;
-  }
-
   _sourceHMRURL(platform: ?string, hmrpath: string) {
     return this._hmrURL('', platform, 'bundle', hmrpath);
   }
@@ -946,18 +943,6 @@ function verifyRootExists(root) {
   assert(fs.statSync(root).isDirectory(), 'Root has to be a valid directory');
 }
 
-function createModuleIdFactory() {
-  const fileToIdMap = Object.create(null);
-  let nextId = 0;
-  return ({path: modulePath}) => {
-    if (!(modulePath in fileToIdMap)) {
-      fileToIdMap[modulePath] = nextId;
-      nextId += 1;
-    }
-    return fileToIdMap[modulePath];
-  };
-}
-
 function getMainModule({dependencies, numPrependedDependencies = 0}) {
   return dependencies[numPrependedDependencies];
 }
diff --git a/packages/metro-bundler/src/DeltaBundler/DeltaTransformer.js b/packages/metro-bundler/src/DeltaBundler/DeltaTransformer.js
index 0c8e6a69..f6b58ec8 100644
--- a/packages/metro-bundler/src/DeltaBundler/DeltaTransformer.js
+++ b/packages/metro-bundler/src/DeltaBundler/DeltaTransformer.js
@@ -14,6 +14,8 @@
 
 const DeltaCalculator = require('./DeltaCalculator');
 
+const createModuleIdFactory = require('../lib/createModuleIdFactory');
+
 const {EventEmitter} = require('events');
 
 import type {RawMapping} from '../Bundler/source-map';
@@ -47,6 +49,8 @@ type Options = {|
   +polyfillModuleNames: $ReadOnlyArray<string>,
 |};
 
+const globalCreateModuleId = createModuleIdFactory();
+
 /**
  * This class is in charge of creating the delta bundle with the actual
  * transformed source code for each of the modified modules. For each modified
@@ -88,9 +92,19 @@ class DeltaTransformer extends EventEmitter {
     this._deltaCalculator = deltaCalculator;
     this._getPolyfills = options.getPolyfills;
     this._polyfillModuleNames = options.polyfillModuleNames;
-    this._getModuleId = this._bundler.getGetModuleIdFn();
     this._bundleOptions = bundleOptions;
 
+    // Only when isolateModuleIDs is true the Module IDs of this instance are
+    // sandboxed from the rest.
+    // Isolating them makes sense when we want to get consistent module IDs
+    // between different builds of the same bundle (for example when building
+    // production builds), while coupling them makes sense when we want
+    // different bundles to share the same ids (on HMR, where we need to patch
+    // the correct module).
+    this._getModuleId = this._bundleOptions.isolateModuleIDs
+      ? createModuleIdFactory()
+      : globalCreateModuleId;
+
     this._deltaCalculator.on('change', this._onFileChange);
   }
 
diff --git a/packages/metro-bundler/src/lib/createModuleIdFactory.js b/packages/metro-bundler/src/lib/createModuleIdFactory.js
new file mode 100644
index 00000000..82cf7d65
--- /dev/null
+++ b/packages/metro-bundler/src/lib/createModuleIdFactory.js
@@ -0,0 +1,26 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @format
+ */
+
+'use strict';
+
+function createModuleIdFactory(): ({path: string}) => number {
+  const fileToIdMap = new Map();
+  let nextId = 0;
+  return ({path: modulePath}) => {
+    if (!fileToIdMap.has(modulePath)) {
+      fileToIdMap.set(modulePath, nextId);
+      nextId += 1;
+    }
+    return fileToIdMap.get(modulePath);
+  };
+}
+
+module.exports = createModuleIdFactory;