``dest`` parameter behaves the same for positional and optional arguments.

   >>> parser = ArgumentParser()
   >>> parser.add_argument('foo-bar')
   >>> args = parser.parse_args(['f'])
   >>> print(args.foo_bar)
   f

Also preserves backwards compatibility for pre-3.4 attribute access:

   >>> print(args.foo_bar == getattr(args, 'foo-bar'))
   True

diff --git a/Lib/argparse.py b/Lib/argparse.py
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -1191,7 +1191,17 @@ class Namespace(_AttributeHolder):
         return not (self == other)
 
     def __contains__(self, key):
-        return key in self.__dict__
+        return key in self.__dict__ or key.replace('-', '_') in self.__dict__
+
+    def __getattr__(self, name):
+        # Compatibility for people doing getattr(args, 'foo-bar') instead of
+        # args.foo_bar. This might lead to some false positives, but this
+        # is unlikely.
+        try:
+            return self.__dict__[name.replace('-', '_')]
+        except KeyError:
+            msg = "'%s' object has no attribute '%s'"
+            raise AttributeError(msg % (type(self).__name__, name))
 
 
 class _ActionsContainer(object):
@@ -1411,6 +1421,9 @@ class _ActionsContainer(object):
         if kwargs.get('nargs') == ZERO_OR_MORE and 'default' not in kwargs:
             kwargs['required'] = True
 
+        # make dest attribute-accessible, 'foo-bar' -> 'foo_bar'
+        dest = dest.replace('-', '_')
+
         # return the keyword arguments with no option strings
         return dict(kwargs, dest=dest, option_strings=[])
 
diff --git a/Lib/test/test_argparse.py b/Lib/test/test_argparse.py
--- a/Lib/test/test_argparse.py
+++ b/Lib/test/test_argparse.py
@@ -1116,6 +1116,27 @@ class TestPositionalsNargsOptionalOneOrM
     ]
 
 
+class TestPositionalDest(ParserTestCase):
+    """Tests setting destination"""
+
+    argument_signatures = [Sig('foo-bar')]
+    failures = []
+    successes = [
+        ('f', NS(foo_bar='f')),
+    ]
+
+    def test_compatibility(self):
+        parser = ErrorRaisingArgumentParser(prog='PROG',
+                                            description='main description')
+        parser.add_argument('foo-bar')
+        args = parser.parse_args(['f'])
+        self.assertEqual(args.foo_bar, 'f')
+        self.assertEqual(getattr(args, 'foo-bar'), 'f')
+        self.assertRaisesRegexp(AttributeError,
+                                "'Namespace' object has no attribute 'a-b'",
+                                getattr, args, 'a-b')
+
+
 class TestPositionalsChoicesString(ParserTestCase):
     """Test a set of single-character choices"""