diff --git a/packages/metro/src/JSTransformer/__tests__/Transformer-test.js b/packages/metro/src/JSTransformer/__tests__/Transformer-test.js
index 0395ba6e..654a3364 100644
--- a/packages/metro/src/JSTransformer/__tests__/Transformer-test.js
+++ b/packages/metro/src/JSTransformer/__tests__/Transformer-test.js
@@ -66,10 +66,10 @@ describe('Transformer', function() {
     );
 
     expect(api.transform).toBeCalledWith(
-      transformModulePath,
       fileName,
       localPath,
       code,
+      transformModulePath,
       false,
       transformOptions,
       [],
@@ -83,7 +83,7 @@ describe('Transformer', function() {
     const snippet = 'snippet';
 
     api.transform.mockImplementation(
-      (transformPath, filename, localPth, code, opts) => {
+      (filename, localPth, code, transformPath, opts) => {
         const babelError = new SyntaxError(message);
 
         babelError.type = 'SyntaxError';
diff --git a/packages/metro/src/JSTransformer/index.js b/packages/metro/src/JSTransformer/index.js
index c3f03454..737d77e1 100644
--- a/packages/metro/src/JSTransformer/index.js
+++ b/packages/metro/src/JSTransformer/index.js
@@ -49,6 +49,7 @@ module.exports = class Transformer {
     if (maxWorkers > 1) {
       this._worker = this._makeFarm(
         workerPath,
+        this._computeWorkerKey,
         ['minify', 'transform'],
         maxWorkers,
       );
@@ -92,10 +93,10 @@ module.exports = class Transformer {
       debug('Started ransforming file', filename);
 
       const data = await this._worker.transform(
-        this._transformModulePath,
         filename,
         localPath,
         code,
+        this._transformModulePath,
         isScript,
         options,
         assetExts,
@@ -119,7 +120,7 @@ module.exports = class Transformer {
     }
   }
 
-  _makeFarm(workerPath, exposedMethods, maxWorkers) {
+  _makeFarm(workerPath, computeWorkerKey, exposedMethods, maxWorkers) {
     // We whitelist only what would work. For example `--inspect` doesn't work
     // in the workers because it tries to open the same debugging port. Feel
     // free to add more cases to the RegExp. A whitelist is preferred, to
@@ -132,12 +133,24 @@ module.exports = class Transformer {
     );
 
     return new Worker(workerPath, {
+      computeWorkerKey,
       exposedMethods,
       forkOptions: {execArgv},
       maxWorkers,
     });
   }
 
+  _computeWorkerKey(method: string, filename: string): ?string {
+    // Only when transforming a file we want to stick to the same worker; and
+    // we'll shard by file path. If not; we return null, which tells the worker
+    // to pick the first available one.
+    if (method === 'transform') {
+      return filename;
+    }
+
+    return null;
+  }
+
   _formatGenericError(err, filename) {
     const error = new TransformError(`${filename}: ${err.message}`);
 
diff --git a/packages/metro/src/JSTransformer/worker/__tests__/worker-test.js b/packages/metro/src/JSTransformer/worker/__tests__/worker-test.js
index 4255306d..92958d82 100644
--- a/packages/metro/src/JSTransformer/worker/__tests__/worker-test.js
+++ b/packages/metro/src/JSTransformer/worker/__tests__/worker-test.js
@@ -23,10 +23,10 @@ const {InvalidRequireCallError} = require('..');
 describe('code transformation worker:', () => {
   it('transforms a simple script', async () => {
     const {result} = await transformCode(
-      path.join(__dirname, '../../../transformer.js'),
       'arbitrary/file.js',
       `local/file.js`,
       'someReallyArbitrary(code)',
+      path.join(__dirname, '../../../transformer.js'),
       true,
       {
         dev: true,
@@ -49,10 +49,10 @@ describe('code transformation worker:', () => {
 
   it('transforms a simple module', async () => {
     const {result} = await transformCode(
-      path.join(__dirname, '../../../transformer.js'),
       'arbitrary/file.js',
       `local/file.js`,
       'arbitrary(code)',
+      path.join(__dirname, '../../../transformer.js'),
       false,
       {
         dev: true,
@@ -75,7 +75,6 @@ describe('code transformation worker:', () => {
 
   it('transforms a module with dependencies', async () => {
     const {result} = await transformCode(
-      path.join(__dirname, '../../../transformer.js'),
       'arbitrary/file.js',
       `local/file.js`,
       [
@@ -84,6 +83,7 @@ describe('code transformation worker:', () => {
         'const b = require("b");',
         'import c from "./c";',
       ].join('\n'),
+      path.join(__dirname, '../../../transformer.js'),
       false,
       {
         dev: true,
@@ -115,7 +115,6 @@ describe('code transformation worker:', () => {
   it('reports filename when encountering unsupported dynamic dependency', async () => {
     try {
       await transformCode(
-        path.join(__dirname, '../../../transformer.js'),
         'arbitrary/file.js',
         `local/file.js`,
         [
@@ -123,6 +122,7 @@ describe('code transformation worker:', () => {
           'let a = arbitrary(code);',
           'const b = require(a);',
         ].join('\n'),
+        path.join(__dirname, '../../../transformer.js'),
         false,
         {
           dev: true,
diff --git a/packages/metro/src/JSTransformer/worker/index.js b/packages/metro/src/JSTransformer/worker/index.js
index 6eabc3bf..0e0adabd 100644
--- a/packages/metro/src/JSTransformer/worker/index.js
+++ b/packages/metro/src/JSTransformer/worker/index.js
@@ -163,10 +163,10 @@ function postTransform(
 }
 
 function transformCode(
-  transformerPath: string,
   filename: string,
   localPath: LocalPath,
   sourceCode: string,
+  transformerPath: string,
   isScript: boolean,
   options: Options,
   assetExts: $ReadOnlyArray<string>,