diff --git a/packages/metro/src/DeltaBundler/Serializers/__tests__/sourceMapString-test.js b/packages/metro/src/DeltaBundler/Serializers/__tests__/sourceMapString-test.js
index baa902b3af..592e7928dd 100644
--- a/packages/metro/src/DeltaBundler/Serializers/__tests__/sourceMapString-test.js
+++ b/packages/metro/src/DeltaBundler/Serializers/__tests__/sourceMapString-test.js
@@ -15,7 +15,10 @@ import type {Module} from '../../types.flow';
 
 import CountingSet from '../../../lib/CountingSet';
 
-const sourceMapString = require('../sourceMapString');
+const {
+  sourceMapString,
+  sourceMapStringNonBlocking,
+} = require('../sourceMapString');
 
 const polyfill: Module<> = {
   path: '/root/pre.js',
@@ -77,102 +80,119 @@ const barModule: Module<> = {
   ],
 };
 
-it('should serialize a very simple bundle', () => {
-  expect(
-    JSON.parse(
-      sourceMapString([polyfill, fooModule, barModule], {
-        excludeSource: false,
-        processModuleFilter: module => true,
-        shouldAddToIgnoreList: module => false,
-        getSourceUrl: null,
-      }),
-    ),
-  ).toEqual({
-    version: 3,
-    sources: ['/root/pre.js', '/root/foo.js', '/root/bar.js'],
-    sourcesContent: ['source pre', 'source foo', 'source bar'],
-    x_facebook_sources: [null, [{names: ['<global>'], mappings: 'AAA'}], null],
-    names: [],
-    mappings: '',
-  });
-});
+describe.each([sourceMapString, sourceMapStringNonBlocking])(
+  '%p',
+  sourceMapStringImpl => {
+    test('should serialize a very simple bundle', async () => {
+      expect(
+        JSON.parse(
+          await sourceMapStringImpl([polyfill, fooModule, barModule], {
+            excludeSource: false,
+            processModuleFilter: module => true,
+            shouldAddToIgnoreList: module => false,
+            getSourceUrl: null,
+          }),
+        ),
+      ).toEqual({
+        version: 3,
+        sources: ['/root/pre.js', '/root/foo.js', '/root/bar.js'],
+        sourcesContent: ['source pre', 'source foo', 'source bar'],
+        x_facebook_sources: [
+          null,
+          [{names: ['<global>'], mappings: 'AAA'}],
+          null,
+        ],
+        names: [],
+        mappings: '',
+      });
+    });
 
-it('modules should appear in their original order', () => {
-  expect(
-    JSON.parse(
-      sourceMapString([polyfill, barModule, fooModule], {
-        excludeSource: false,
-        processModuleFilter: module => true,
-        shouldAddToIgnoreList: module => false,
-        getSourceUrl: null,
-      }),
-    ),
-  ).toEqual({
-    version: 3,
-    sources: ['/root/pre.js', '/root/bar.js', '/root/foo.js'],
-    sourcesContent: ['source pre', 'source bar', 'source foo'],
-    x_facebook_sources: [null, null, [{names: ['<global>'], mappings: 'AAA'}]],
-    names: [],
-    mappings: '',
-  });
-});
+    test('modules should appear in their original order', async () => {
+      expect(
+        JSON.parse(
+          await sourceMapStringImpl([polyfill, barModule, fooModule], {
+            excludeSource: false,
+            processModuleFilter: module => true,
+            shouldAddToIgnoreList: module => false,
+            getSourceUrl: null,
+          }),
+        ),
+      ).toEqual({
+        version: 3,
+        sources: ['/root/pre.js', '/root/bar.js', '/root/foo.js'],
+        sourcesContent: ['source pre', 'source bar', 'source foo'],
+        x_facebook_sources: [
+          null,
+          null,
+          [{names: ['<global>'], mappings: 'AAA'}],
+        ],
+        names: [],
+        mappings: '',
+      });
+    });
 
-it('should not include the source of an asset', () => {
-  const assetModule: Module<> = {
-    path: '/root/asset.jpg',
-    dependencies: new Map(),
-    inverseDependencies: new CountingSet(),
-    getSource: () => {
-      throw new Error('should not read the source of an asset');
-    },
-    output: [
-      {
-        type: 'js/module/asset',
-        data: {
-          code: '__d(function() {/* code for bar */});',
-          lineCount: 1,
-          map: [],
+    test('should not include the source of an asset', async () => {
+      const assetModule: Module<> = {
+        path: '/root/asset.jpg',
+        dependencies: new Map(),
+        inverseDependencies: new CountingSet(),
+        getSource: () => {
+          throw new Error('should not read the source of an asset');
         },
-      },
-    ],
-  };
+        output: [
+          {
+            type: 'js/module/asset',
+            data: {
+              code: '__d(function() {/* code for bar */});',
+              lineCount: 1,
+              map: [],
+            },
+          },
+        ],
+      };
 
-  expect(
-    JSON.parse(
-      sourceMapString([fooModule, assetModule], {
-        excludeSource: false,
-        processModuleFilter: module => true,
-        shouldAddToIgnoreList: module => false,
-        getSourceUrl: null,
-      }),
-    ),
-  ).toEqual({
-    version: 3,
-    sources: ['/root/foo.js', '/root/asset.jpg'],
-    sourcesContent: ['source foo', ''],
-    x_facebook_sources: [[{names: ['<global>'], mappings: 'AAA'}], null],
-    names: [],
-    mappings: '',
-  });
-});
+      expect(
+        JSON.parse(
+          await sourceMapStringImpl([fooModule, assetModule], {
+            excludeSource: false,
+            processModuleFilter: module => true,
+            shouldAddToIgnoreList: module => false,
+            getSourceUrl: null,
+          }),
+        ),
+      ).toEqual({
+        version: 3,
+        sources: ['/root/foo.js', '/root/asset.jpg'],
+        sourcesContent: ['source foo', ''],
+        x_facebook_sources: [[{names: ['<global>'], mappings: 'AAA'}], null],
+        names: [],
+        mappings: '',
+      });
+    });
 
-it('should emit x_google_ignoreList based on shouldAddToIgnoreList', () => {
-  expect(
-    JSON.parse(
-      sourceMapString([polyfill, fooModule, barModule], {
-        excludeSource: false,
-        processModuleFilter: module => true,
-        shouldAddToIgnoreList: module => true,
-        getSourceUrl: null,
-      }),
-    ),
-  ).toEqual({
-    version: 3,
-    sources: ['/root/pre.js', '/root/foo.js', '/root/bar.js'],
-    sourcesContent: ['source pre', 'source foo', 'source bar'],
-    x_facebook_sources: [null, [{names: ['<global>'], mappings: 'AAA'}], null],
-    names: [],
-    mappings: '',
-    x_google_ignoreList: [0, 1, 2],
-  });
-});
+    test('should emit x_google_ignoreList based on shouldAddToIgnoreList', async () => {
+      expect(
+        JSON.parse(
+          await sourceMapStringImpl([polyfill, fooModule, barModule], {
+            excludeSource: false,
+            processModuleFilter: module => true,
+            shouldAddToIgnoreList: module => true,
+            getSourceUrl: null,
+          }),
+        ),
+      ).toEqual({
+        version: 3,
+        sources: ['/root/pre.js', '/root/foo.js', '/root/bar.js'],
+        sourcesContent: ['source pre', 'source foo', 'source bar'],
+        x_facebook_sources: [
+          null,
+          [{names: ['<global>'], mappings: 'AAA'}],
+          null,
+        ],
+        names: [],
+        mappings: '',
+        x_google_ignoreList: [0, 1, 2],
+      });
+    });
+  },
+);
diff --git a/packages/metro/src/DeltaBundler/Serializers/sourceMapString.js b/packages/metro/src/DeltaBundler/Serializers/sourceMapString.js
index ee6c9dfab7..d755a0599d 100644
--- a/packages/metro/src/DeltaBundler/Serializers/sourceMapString.js
+++ b/packages/metro/src/DeltaBundler/Serializers/sourceMapString.js
@@ -14,7 +14,10 @@
 import type {Module} from '../types.flow';
 import type {SourceMapGeneratorOptions} from './sourceMapGenerator';
 
-const {sourceMapGenerator} = require('./sourceMapGenerator');
+const {
+  sourceMapGenerator,
+  sourceMapGeneratorNonBlocking,
+} = require('./sourceMapGenerator');
 
 function sourceMapString(
   modules: $ReadOnlyArray<Module<>>,
@@ -25,4 +28,17 @@ function sourceMapString(
   });
 }
 
-module.exports = sourceMapString;
+async function sourceMapStringNonBlocking(
+  modules: $ReadOnlyArray<Module<>>,
+  options: SourceMapGeneratorOptions,
+): Promise<string> {
+  const generator = await sourceMapGeneratorNonBlocking(modules, options);
+  return generator.toString(undefined, {
+    excludeSource: options.excludeSource,
+  });
+}
+
+module.exports = {
+  sourceMapString,
+  sourceMapStringNonBlocking,
+};
diff --git a/packages/metro/src/Server.js b/packages/metro/src/Server.js
index 3db4fe62ff..ce94a10fe9 100644
--- a/packages/metro/src/Server.js
+++ b/packages/metro/src/Server.js
@@ -53,7 +53,9 @@ const {
   getExplodedSourceMap,
 } = require('./DeltaBundler/Serializers/getExplodedSourceMap');
 const getRamBundleInfo = require('./DeltaBundler/Serializers/getRamBundleInfo');
-const sourceMapString = require('./DeltaBundler/Serializers/sourceMapString');
+const {
+  sourceMapStringNonBlocking,
+} = require('./DeltaBundler/Serializers/sourceMapString');
 const IncrementalBundler = require('./IncrementalBundler');
 const ResourceNotFoundError = require('./IncrementalBundler/ResourceNotFoundError');
 const bundleToString = require('./lib/bundleToString');
@@ -288,7 +290,7 @@ class Server {
       ).code;
     }
     if (!bundleMap) {
-      bundleMap = sourceMapString(
+      bundleMap = await sourceMapStringNonBlocking(
         [...prepend, ...this._getSortedModules(graph)],
         {
           excludeSource: serializerOptions.excludeSource,
@@ -1151,14 +1153,17 @@ class Server {
         prepend = [];
       }
 
-      return sourceMapString([...prepend, ...this._getSortedModules(graph)], {
-        excludeSource: serializerOptions.excludeSource,
-        processModuleFilter: this._config.serializer.processModuleFilter,
-        shouldAddToIgnoreList: (module: Module<>) =>
-          this._shouldAddModuleToIgnoreList(module),
-        getSourceUrl: (module: Module<>) =>
-          this._getModuleSourceUrl(module, serializerOptions.sourcePaths),
-      });
+      return await sourceMapStringNonBlocking(
+        [...prepend, ...this._getSortedModules(graph)],
+        {
+          excludeSource: serializerOptions.excludeSource,
+          processModuleFilter: this._config.serializer.processModuleFilter,
+          shouldAddToIgnoreList: (module: Module<>) =>
+            this._shouldAddModuleToIgnoreList(module),
+          getSourceUrl: (module: Module<>) =>
+            this._getModuleSourceUrl(module, serializerOptions.sourcePaths),
+        },
+      );
     },
     finish({mres, result}) {
       mres.setHeader('Content-Type', 'application/json');
diff --git a/packages/metro/src/lib/getAppendScripts.js b/packages/metro/src/lib/getAppendScripts.js
index 730486dbdb..4e7ea7bc29 100644
--- a/packages/metro/src/lib/getAppendScripts.js
+++ b/packages/metro/src/lib/getAppendScripts.js
@@ -16,7 +16,9 @@ import type {Dependency} from '../DeltaBundler/types.flow';
 import CountingSet from './CountingSet';
 
 const getInlineSourceMappingURL = require('../DeltaBundler/Serializers/helpers/getInlineSourceMappingURL');
-const sourceMapString = require('../DeltaBundler/Serializers/sourceMapString');
+const {
+  sourceMapString,
+} = require('../DeltaBundler/Serializers/sourceMapString');
 const countLines = require('./countLines');
 const nullthrows = require('nullthrows');