diff --git a/lib/6to5/transformers/_use-strict.js b/lib/6to5/transformers/_use-strict.js
new file mode 100644
index 0000000000..11d682480a
--- /dev/null
+++ b/lib/6to5/transformers/_use-strict.js
@@ -0,0 +1,15 @@
+var b = require("recast").types.builders;
+
+module.exports = function (ast, file) {
+  var body = ast.program.body;
+  var first = body[0];
+
+  var noStrict = !first || first.type !== "ExpressionStatement" || first.expression.type !== "Literal" || first.expression.value !== "use strict";
+
+  if (noStrict) {
+    if (file.opts._noStrict) return;
+    body.unshift(b.expressionStatement(b.literal("use strict")));
+  } else {
+    if (file.opts._noStrict) body.shift();
+  }
+};