diff --git a/pyflakes/checker.py b/pyflakes/checker.py
index f1eff00..f00e909 100644
--- a/pyflakes/checker.py
+++ b/pyflakes/checker.py
@@ -3,7 +3,7 @@
 # See LICENSE file for details
 
 import __builtin__
-import os.path
+import os
 import _ast
 
 from pyflakes import messages
@@ -164,10 +164,6 @@ class ModuleScope(Scope):
     pass
 
 
-# Globally defined names which are not attributes of the __builtin__ module.
-_MAGIC_GLOBALS = ['__file__', '__builtins__']
-
-
 
 class Checker(object):
     """
@@ -186,6 +182,12 @@ class Checker(object):
     traceTree = False
 
     def __init__(self, tree, filename=None):
+        # Globally defined names which are not attributes of the
+        # __builtin__ module.
+        self.MAGIC_GLOBALS = (
+            ['__file__', '__builtins__'] +
+            os.environ.get('PYFLAKES_GLOBALS_WHITELIST', '').split(','))
+
         if filename is None:
             filename = '(none)'
         self._deferredFunctions = []
@@ -464,7 +466,7 @@ def NAME(self, node):
                 self.scopeStack[0][node.id].used = (self.scope, node)
             except KeyError:
                 if ((not hasattr(__builtin__, node.id))
-                        and node.id not in _MAGIC_GLOBALS
+                        and node.id not in self.MAGIC_GLOBALS
                         and not importStarred):
                     if (os.path.basename(self.filename) == '__init__.py' and
                         node.id == '__path__'):
diff --git a/pyflakes/test/test_undefined_names.py b/pyflakes/test/test_undefined_names.py
index 309f0b9..1457454 100644
--- a/pyflakes/test/test_undefined_names.py
+++ b/pyflakes/test/test_undefined_names.py
@@ -1,4 +1,4 @@
-
+import os
 from _ast import PyCF_ONLY_AST
 
 from twisted.trial.unittest import TestCase
@@ -43,6 +43,25 @@ def test_magicGlobalsBuiltins(self):
         self.flakes('__builtins__')
 
 
+    def test_magicGlobalsWhitelistSingle(self):
+        """
+        Use of a whitelisted magic global should not emit an undefined
+        name warning.
+        """
+        os.environ['PYFLAKES_GLOBALS_WHITELIST'] = '_'
+        self.flakes('_')
+
+
+    def test_magicGlobalsWhitelistMultiple(self):
+        """
+        Use of whitelisted magic globals should not emit an undefined
+        name warning.
+        """
+        os.environ['PYFLAKES_GLOBALS_WHITELIST'] = '_,whitelisted'
+        self.flakes('_')
+        self.flakes('whitelisted')
+
+
     def test_magicGlobalsName(self):
         """
         Use of the C{__name__} magic global should not emit an undefined name