diff --git a/NEWS.txt b/NEWS.txt
index 56c251c1..ec6bffe2 100644
--- a/NEWS.txt
+++ b/NEWS.txt
@@ -3,6 +3,11 @@ UNRELEASED:
   - Fix caret position on SyntaxError.
   - Fix crash on Python 2.x with some doctest SyntaxError.
   - Add tox.ini.
+  - The `PYFLAKES_NODOCTEST` environment variable has been replaced with the
+    `PYFLAKES_DOCTEST` environment variable (with the opposite meaning).
+    Doctest checking is now disabled by default; set the environment variable
+    to enable it.
+
 
 0.7.3 (2013-07-02):
   - Do not report undefined name for generator expression and dict or
diff --git a/pyflakes/checker.py b/pyflakes/checker.py
index 04491f9f..1cab12e0 100644
--- a/pyflakes/checker.py
+++ b/pyflakes/checker.py
@@ -231,7 +231,6 @@ class Checker(object):
     nodeDepth = 0
     offset = None
     traceTree = False
-    withDoctest = ('PYFLAKES_NODOCTEST' not in os.environ)
 
     builtIns = set(builtin_vars).union(_MAGIC_GLOBALS)
     _customBuiltIns = os.environ.get('PYFLAKES_BUILTINS')
@@ -239,7 +238,8 @@ class Checker(object):
         builtIns.update(_customBuiltIns.split(','))
     del _customBuiltIns
 
-    def __init__(self, tree, filename='(none)', builtins=None):
+    def __init__(self, tree, filename='(none)', builtins=None,
+                 withDoctest='PYFLAKES_DOCTEST' in os.environ):
         self._nodeHandlers = {}
         self._deferredFunctions = []
         self._deferredAssignments = []
@@ -248,6 +248,7 @@ def __init__(self, tree, filename='(none)', builtins=None):
         self.filename = filename
         if builtins:
             self.builtIns = self.builtIns.union(builtins)
+        self.withDoctest = withDoctest
         self.scopeStack = [ModuleScope()]
         self.exceptHandlers = [()]
         self.futuresAllowed = True
diff --git a/pyflakes/test/harness.py b/pyflakes/test/harness.py
index 9f337e58..a781237d 100644
--- a/pyflakes/test/harness.py
+++ b/pyflakes/test/harness.py
@@ -18,9 +18,11 @@
 
 class TestCase(unittest.TestCase):
 
+    withDoctest = False
+
     def flakes(self, input, *expectedOutputs, **kw):
         tree = compile(textwrap.dedent(input), "<test>", "exec", PyCF_ONLY_AST)
-        w = checker.Checker(tree, **kw)
+        w = checker.Checker(tree, withDoctest=self.withDoctest, **kw)
         outputs = [type(o) for o in w.messages]
         expectedOutputs = list(expectedOutputs)
         outputs.sort(key=lambda t: t.__name__)
diff --git a/pyflakes/test/test_doctests.py b/pyflakes/test/test_doctests.py
index d7df7792..8ff96203 100644
--- a/pyflakes/test/test_doctests.py
+++ b/pyflakes/test/test_doctests.py
@@ -9,6 +9,8 @@
 
 class Test(TestOther, TestImports, TestUndefinedNames):
 
+    withDoctest = True
+
     def doctestify(self, input):
         lines = []
         for line in textwrap.dedent(input).splitlines():