diff --git a/lib/babel.js b/lib/babel.js
index bdff5e78db..390b1599f8 100644
--- a/lib/babel.js
+++ b/lib/babel.js
@@ -1,10 +1,13 @@
 'use strict';
+var path = require('path');
 var resolveFrom = require('resolve-from');
 var createEspowerPlugin = require('babel-plugin-espower/create');
 var requireFromString = require('require-from-string');
 
 var hasGenerators = parseInt(process.version.slice(1), 10) > 0;
 var testPath = process.argv[2];
+var testDir = path.dirname(testPath);
+var Module = module.constructor;
 var babel;
 
 try {
@@ -14,17 +17,59 @@ try {
 	babel = require('babel-core');
 }
 
-var options = {
-	blacklist: hasGenerators ? ['regenerator'] : [],
-	optional: hasGenerators ? ['asyncToGenerator', 'runtime'] : ['runtime'],
-	plugins: [
-		createEspowerPlugin(babel, {
-			patterns: require('./enhance-assert').PATTERNS
-		})
-	]
-};
-
-var transpiled = babel.transformFileSync(testPath, options);
-requireFromString(transpiled.code, testPath, {
-	appendPaths: module.paths
-});
+function options() {
+	return {
+		blacklist: hasGenerators ? ['regenerator'] : [],
+		optional: hasGenerators ? ['asyncToGenerator', 'runtime'] : ['runtime'],
+		plugins: [
+			createEspowerPlugin(babel, {
+				patterns: require('./enhance-assert').PATTERNS
+			})
+		]
+	};
+}
+
+// TODO: This needs to be smarter.
+function shouldTranspile(filePath) {
+	var fileName = path.basename(filePath);
+	return testDir === path.dirname(filePath) && fileName[0] === '_' || filePath === testPath;
+}
+
+function requireHook(file) {
+	// Most of this is Module._load with a few modifications.
+
+	file = Module._resolveFilename(file, this);
+
+	var cachedModule = Module._cache[file];
+	if (cachedModule) {
+		return cachedModule.exports;
+	}
+
+	if (shouldTranspile(file)) {
+		var code = babel.transformFileSync(file, options()).code;
+
+		var hadException = true;
+
+		var m = requireFromString.load(file, {
+			appendPaths: module.paths,
+			require: requireHook
+		});
+
+		try {
+			Module._cache[file] = m;
+
+			m._compile(code, file);
+
+			hadException = false;
+		} finally {
+			if (hadException) {
+				delete Module._cache[file];
+			}
+		}
+
+		return m.exports;
+	}
+	return Module._load(file, this);
+}
+
+requireHook.call(module, testPath);
diff --git a/lib/fork.js b/lib/fork.js
index 8fadb88054..9160731e48 100644
--- a/lib/fork.js
+++ b/lib/fork.js
@@ -33,7 +33,7 @@ module.exports = function (args) {
 
 		ps.on('exit', function (code) {
 			if (code > 0 && code !== 143) {
-				reject();
+				reject(new Error(file + ' exited with error code ' + code));
 			} else {
 				resolve(testResults);
 			}
diff --git a/package.json b/package.json
index 36f967c38f..22e98820ea 100644
--- a/package.json
+++ b/package.json
@@ -81,7 +81,7 @@
     "power-assert-formatter": "^1.3.0",
     "power-assert-renderers": "^0.1.0",
     "pretty-ms": "^2.0.0",
-    "require-from-string": "^1.1.0",
+    "require-from-string": "jamestalmage/require-from-string#allow-transforms",
     "resolve-from": "^1.0.0",
     "serialize-error": "^1.0.0",
     "set-immediate-shim": "^1.0.1",
diff --git a/test/fixture/_es6-helper.js b/test/fixture/_es6-helper.js
new file mode 100644
index 0000000000..50951f5ec7
--- /dev/null
+++ b/test/fixture/_es6-helper.js
@@ -0,0 +1,5 @@
+'use strict';
+
+export default function () {
+	return Promise.resolve('es6').then(prefix => prefix + ' helper');
+}
diff --git a/test/fixture/babel-hook-imported.js b/test/fixture/babel-hook-imported.js
index 121e27f6a8..81aff321c0 100644
--- a/test/fixture/babel-hook-imported.js
+++ b/test/fixture/babel-hook-imported.js
@@ -1 +1 @@
-module.export = async () => {};
+module.exports = async () => {};
diff --git a/test/fixture/uses-es6-helper.js b/test/fixture/uses-es6-helper.js
new file mode 100644
index 0000000000..a00dcb828a
--- /dev/null
+++ b/test/fixture/uses-es6-helper.js
@@ -0,0 +1,9 @@
+'use strict';
+import test from '../../';
+
+import helper from './_es6-helper';
+
+test(async t => {
+	t.is(await helper(), 'es6 helper');
+	t.end();
+});
diff --git a/test/fork.js b/test/fork.js
index a8922735c5..56988cfee9 100644
--- a/test/fork.js
+++ b/test/fork.js
@@ -35,7 +35,7 @@ test('rejects on error and streams output', function (t) {
 			buffer += data;
 		})
 		.catch(function () {
-			t.ok(/no such file or directory/.test(buffer));
+			t.ok(/Cannot find module/.test(buffer));
 			t.end();
 		});
 });
diff --git a/test/test.js b/test/test.js
index a25f30c98b..7370703d50 100644
--- a/test/test.js
+++ b/test/test.js
@@ -963,10 +963,27 @@ test('change process.cwd() to a test\'s directory', function (t) {
 	});
 });
 
-test('Babel require hook only applies to the test file', function (t) {
-	execCli('fixture/babel-hook.js', function (err) {
+test('Babel require hook does apply to helpers ("_" prefix)', function (t) {
+	t.plan(1);
+
+	execCli('fixture/uses-es6-helper.js', function (err) {
+		t.ifError(err);
+		t.end();
+	});
+});
+
+test('Babel require does not apply to non-helper (no "_" prefix)', function (t) {
+	execCli('fixture/babel-hook.js', function (err, stdout) {
+		// TODO: This test was passing for the wrong reasons. Make sure this gets fixed in master regardless.
 		t.ok(err);
 		t.is(err.code, 1);
+
+		// module.exports = async () => {};
+		//                            ^
+		// SyntaxError: Unexpected reserved word
+
+		// TODO: Why standard error?
+		t.true(/module.exports = async \(\) => \{\};\s*\n.*?\n.*?Unexpected token >/.test(stdout));
 		t.end();
 	});
 });