diff --git a/lib/build/dependency-inclusion.js b/lib/build/dependency-inclusion.js
index 8685fb37..c231403d 100644
--- a/lib/build/dependency-inclusion.js
+++ b/lib/build/dependency-inclusion.js
@@ -28,7 +28,7 @@ exports.DependencyInclusion = class {
     }
 
     let main = this.description.loaderConfig.main;
-    if (mainIsJs) {
+    if (mainIsJs && Utils.knownExtensions.indexOf(ext) === -1) {
       main += '.js';
     }
 
diff --git a/lib/build/find-deps.js b/lib/build/find-deps.js
index 2d2ef198..eb84f8f9 100644
--- a/lib/build/find-deps.js
+++ b/lib/build/find-deps.js
@@ -288,7 +288,7 @@ exports.findHtmlDeps = function(filename, contents, loaderType = 'require') {
 exports.findDeps = function(filename, contents, loaderType = 'require') {
   let ext = path.extname(filename).toLowerCase();
 
-  if (ext === '.js') {
+  if (ext === '.js' || ext === '.mjs' || ext === '.cjs') {
     return exports.findJsDeps(filename, contents, loaderType);
   } else if (ext === '.html' || ext === '.htm') {
     return exports.findHtmlDeps(filename, contents, loaderType);
diff --git a/spec/lib/build/dependency-inclusion.spec.js b/spec/lib/build/dependency-inclusion.spec.js
index bab2fce5..a72e9856 100644
--- a/spec/lib/build/dependency-inclusion.spec.js
+++ b/spec/lib/build/dependency-inclusion.spec.js
@@ -103,6 +103,36 @@ describe('the DependencyInclusion module', () => {
       .catch(e => done.fail(e));
   });
 
+  it('adds mjs main file to the bundle when there is a main file', done => {
+    let bundle = {
+      bundler: bundler,
+      addAlias: jasmine.createSpy('addAlias'),
+      includes: [],
+      createMatcher: function(pattern) {
+        return new Minimatch(pattern, {
+          dot: true
+        });
+      }
+    };
+
+    let description = new DependencyDescription('my-package', 'npm');
+    description.loaderConfig = {
+      path: '../node_modules/my-package',
+      name: 'my-package',
+      main: 'index.mjs'
+    };
+
+    let sut = new DependencyInclusion(bundle, description);
+    sut.traceResources()
+      .then(() => {
+        expect(bundle.includes.length).toBe(1);
+        expect(bundle.includes[0].pattern).toBe(path.join('..', 'node_modules', 'my-package', 'index.mjs'));
+        expect(bundle.addAlias).toHaveBeenCalledWith('my-package', 'my-package/index.mjs');
+        done();
+      })
+      .catch(e => done.fail(e));
+  });
+
   it('aliases main file when both package name and main file share same non-js extension', done => {
     let bundle = {
       bundler: bundler,
diff --git a/spec/lib/build/find-deps.spec.js b/spec/lib/build/find-deps.spec.js
index 31f55ca3..f0efbe35 100644
--- a/spec/lib/build/find-deps.spec.js
+++ b/spec/lib/build/find-deps.spec.js
@@ -71,6 +71,9 @@ describe('find-deps', () => {
       expect(findJsDeps('ignore.js', contents).sort())
         .toEqual(['./b/c', 'a']);
 
+      expect(findJsDeps('ignore.mjs', "define(['./b.mjs'], () => 1);").sort())
+      .toEqual(['./b.mjs']);
+
       expect(findJsDeps('ignore.js', 'define(() => 1);').length).toBe(0);
     });