diff --git a/src/Bundle.js b/src/Bundle.js
index e81c190..d5163d0 100644
--- a/src/Bundle.js
+++ b/src/Bundle.js
@@ -1,4 +1,4 @@
-import Promise from 'es6-promise/lib/es6-promise/promise';
+import Promise from 'es6-promise/lib/es6-promise/promise.js';
 import MagicString from 'magic-string';
 import first from './utils/first.js';
 import { blank, keys } from './utils/object.js';
diff --git a/src/Module.js b/src/Module.js
index cca6854..0940ad2 100644
--- a/src/Module.js
+++ b/src/Module.js
@@ -1,4 +1,4 @@
-import { parse } from 'acorn/src/index';
+import { parse } from 'acorn/src/index.js';
 import MagicString from 'magic-string';
 import { walk } from 'estree-walker';
 import Statement from './Statement.js';
diff --git a/src/ast/Scope.js b/src/ast/Scope.js
index df082e9..7a210a4 100644
--- a/src/ast/Scope.js
+++ b/src/ast/Scope.js
@@ -1,4 +1,4 @@
-import { blank, keys } from '../utils/object';
+import { blank, keys } from '../utils/object.js';
 
 const extractors = {
 	Identifier ( names, param ) {
diff --git a/src/finalisers/amd.js b/src/finalisers/amd.js
index 4067aab..7f63305 100644
--- a/src/finalisers/amd.js
+++ b/src/finalisers/amd.js
@@ -1,4 +1,4 @@
-import { getName, quoteId } from '../utils/map-helpers';
+import { getName, quoteId } from '../utils/map-helpers.js';
 import getInteropBlock from './shared/getInteropBlock.js';
 import getExportBlock from './shared/getExportBlock.js';
 
diff --git a/src/finalisers/es6.js b/src/finalisers/es6.js
index 0f6d864..f64f978 100644
--- a/src/finalisers/es6.js
+++ b/src/finalisers/es6.js
@@ -1,4 +1,4 @@
-import { keys } from '../utils/object';
+import { keys } from '../utils/object.js';
 
 function notDefault ( name ) {
 	return name !== 'default';
diff --git a/src/finalisers/iife.js b/src/finalisers/iife.js
index e8645a1..9236821 100644
--- a/src/finalisers/iife.js
+++ b/src/finalisers/iife.js
@@ -1,5 +1,5 @@
-import { blank } from '../utils/object';
-import { getName } from '../utils/map-helpers';
+import { blank } from '../utils/object.js';
+import { getName } from '../utils/map-helpers.js';
 import getInteropBlock from './shared/getInteropBlock.js';
 import getExportBlock from './shared/getExportBlock.js';
 
diff --git a/src/finalisers/umd.js b/src/finalisers/umd.js
index 00d4cdb..85fba7b 100644
--- a/src/finalisers/umd.js
+++ b/src/finalisers/umd.js
@@ -1,5 +1,5 @@
-import { blank } from '../utils/object';
-import { getName, quoteId, req } from '../utils/map-helpers';
+import { blank } from '../utils/object.js';
+import { getName, quoteId, req } from '../utils/map-helpers.js';
 import getInteropBlock from './shared/getInteropBlock.js';
 import getExportBlock from './shared/getExportBlock.js';
 
diff --git a/src/utils/first.js b/src/utils/first.js
index f703735..0fe2587 100644
--- a/src/utils/first.js
+++ b/src/utils/first.js
@@ -1,4 +1,4 @@
-import Promise from 'es6-promise/lib/es6-promise/promise';
+import Promise from 'es6-promise/lib/es6-promise/promise.js';
 
 // Return the first non-falsy result from an array of
 // maybe-sync, maybe-promise-returning functions
diff --git a/src/utils/fs.js b/src/utils/fs.js
index d0f8715..7a24c24 100644
--- a/src/utils/fs.js
+++ b/src/utils/fs.js
@@ -1,4 +1,4 @@
-import Promise from 'es6-promise/lib/es6-promise/promise';
+import Promise from 'es6-promise/lib/es6-promise/promise.js';
 import * as fs from 'fs';
 import { dirname } from './path.js';