diff --git a/modules/@angular/core/src/application_module.ts b/modules/@angular/core/src/application_module.ts
index 92c3d5855b..123b716043 100644
--- a/modules/@angular/core/src/application_module.ts
+++ b/modules/@angular/core/src/application_module.ts
@@ -40,7 +40,7 @@ export function _keyValueDiffersFactory() {
     ViewUtils,
     {provide: IterableDiffers, useFactory: _iterableDiffersFactory},
     {provide: KeyValueDiffers, useFactory: _keyValueDiffersFactory},
-    {provide: LOCALE_ID, useValue: 'en_US'},
+    {provide: LOCALE_ID, useValue: 'en-US'},
   ]
 })
 export class ApplicationModule {
diff --git a/modules/@angular/core/test/application_module_spec.ts b/modules/@angular/core/test/application_module_spec.ts
index 49ec8e1cf2..80f69a96e3 100644
--- a/modules/@angular/core/test/application_module_spec.ts
+++ b/modules/@angular/core/test/application_module_spec.ts
@@ -11,7 +11,7 @@ import {beforeEach, beforeEachProviders, ddescribe, describe, expect, iit, injec
 
 export function main() {
   describe('Application module', () => {
-    it('should set the default locale to "en_US"',
-       inject([LOCALE_ID], (defaultLocale: string) => { expect(defaultLocale).toEqual('en_US'); }));
+    it('should set the default locale to "en-US"',
+       inject([LOCALE_ID], (defaultLocale: string) => { expect(defaultLocale).toEqual('en-US'); }));
   });
 }
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