diff --git a/packages/metro-bundler/src/JSTransformer/index.js b/packages/metro-bundler/src/JSTransformer/index.js
index 22d75daf..081a0489 100644
--- a/packages/metro-bundler/src/JSTransformer/index.js
+++ b/packages/metro-bundler/src/JSTransformer/index.js
@@ -51,7 +51,7 @@ function makeFarm(worker, methods, timeout, maxConcurrentWorkers) {
 
 class Transformer {
 
-  _workers: {[name: string]: mixed};
+  _workers: {[name: string]: Function};
   _transformModulePath: string;
   _transform: (
     transform: string,
diff --git a/packages/metro-bundler/src/worker-farm/lib/farm.js b/packages/metro-bundler/src/worker-farm/lib/farm.js
index 4624edf4..3bf82848 100644
--- a/packages/metro-bundler/src/worker-farm/lib/farm.js
+++ b/packages/metro-bundler/src/worker-farm/lib/farm.js
@@ -5,6 +5,8 @@
  * This source code is licensed under the BSD-style license found in the
  * LICENSE file in the root directory of this source tree. An additional grant
  * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @flow
  */
 
 /* eslint-disable */
@@ -25,7 +27,7 @@ const extend                  = require('xtend')
     , ProcessTerminatedError  = require('errno').create('ProcessTerminatedError')
     , MaxConcurrentCallsError = require('errno').create('MaxConcurrentCallsError')
 
-function Farm (options, path) {
+function Farm (options: {}, path: string) {
   this.options     = extend(DEFAULT_OPTIONS, options)
   this.path        = path
   this.activeCalls = 0
diff --git a/packages/metro-bundler/src/worker-farm/lib/fork.js b/packages/metro-bundler/src/worker-farm/lib/fork.js
index 63d19d1e..a246df6b 100644
--- a/packages/metro-bundler/src/worker-farm/lib/fork.js
+++ b/packages/metro-bundler/src/worker-farm/lib/fork.js
@@ -5,31 +5,34 @@
  * This source code is licensed under the BSD-style license found in the
  * LICENSE file in the root directory of this source tree. An additional grant
  * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @flow
  */
 
-/* eslint-disable */
-const childProcess = require('child_process')
-    , childModule  = require.resolve('./child/index')
+'use strict';
 
-function fork (forkModule) {
-  var child = childProcess.fork(childModule, {
-          env: process.env
-        , cwd: process.cwd()
-      })
+const childProcess = require('child_process');
+const childModule = require.resolve('./child/index');
 
-  child.send({ module: forkModule })
+function fork(forkModule: string) {
+  const child = childProcess.fork(childModule, {
+    env: process.env,
+    cwd: process.cwd(),
+  });
+
+  child.send({module: forkModule});
 
   // return a send() function for this child
   return {
-      send  : function (data) {
-        try {
-          child.send(data)
-        } catch (e) {
-          // this *should* be picked up by onExit and the operation requeued
-        }
+    send(data: {}) {
+      try {
+        child.send(data);
+      } catch (e) {
+        // this *should* be picked up by onExit and the operation requeued
       }
-    , child : child
-  }
+    },
+    child,
+  };
 }
 
-module.exports = fork
+module.exports = fork;
diff --git a/packages/metro-bundler/src/worker-farm/lib/index.js b/packages/metro-bundler/src/worker-farm/lib/index.js
index 19cd3508..73bd6fa5 100644
--- a/packages/metro-bundler/src/worker-farm/lib/index.js
+++ b/packages/metro-bundler/src/worker-farm/lib/index.js
@@ -5,6 +5,8 @@
  * This source code is licensed under the BSD-style license found in the
  * LICENSE file in the root directory of this source tree. An additional grant
  * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @flow
  */
 
 /* eslint-disable */
@@ -12,20 +14,14 @@ const Farm = require('./farm')
 
 var farms = [] // keep record of farms so we can end() them if required
 
-function farm (options, path, methods) {
-  if (typeof options == 'string') {
-    methods = path
-    path = options
-    options = {}
-  }
-
+function farm(options: {}, path: string, methods: Array<string>): {[name: string]: Function} {
   var f   = new Farm(options, path)
     , api = f.setup(methods)
 
   farms.push({ farm: f, api: api })
 
   // return the public API
-  return api
+  return (api: any)
 }
 
 function end (api, callback) {