diff --git a/spec/babel-spec.coffee b/spec/babel-spec.coffee
index 400e5c03e..9e9195783 100644
--- a/spec/babel-spec.coffee
+++ b/spec/babel-spec.coffee
@@ -42,6 +42,11 @@ describe "Babel transpiler support", ->
       transpiled = require('./fixtures/babel/flow-comment.js')
       expect(transpiled(3)).toBe 4
 
+  describe 'when a .js file starts with // @flow', ->
+    it "transpiles it using babel", ->
+      transpiled = require('./fixtures/babel/flow-slash-comment.js')
+      expect(transpiled(3)).toBe 4
+
   describe "when a .js file does not start with 'use babel';", ->
     it "does not transpile it using babel", ->
       spyOn(console, 'error')
diff --git a/spec/fixtures/babel/flow-slash-comment.js b/spec/fixtures/babel/flow-slash-comment.js
new file mode 100644
index 000000000..b73cc7833
--- /dev/null
+++ b/spec/fixtures/babel/flow-slash-comment.js
@@ -0,0 +1,4 @@
+// @flow
+
+const f: Function = v => v + 1
+module.exports = f
diff --git a/src/babel.js b/src/babel.js
index a944f2e8c..8476a33c0 100644
--- a/src/babel.js
+++ b/src/babel.js
@@ -11,7 +11,8 @@ var PREFIXES = [
   '/** @babel */',
   '"use babel"',
   '\'use babel\'',
-  '/* @flow */'
+  '/* @flow */',
+  '// @flow'
 ]
 
 var PREFIX_LENGTH = Math.max.apply(Math, PREFIXES.map(function (prefix) {