diff -r 8002f45377d4 Lib/multiprocessing/__init__.py
--- a/Lib/multiprocessing/__init__.py	Sat Mar 16 15:39:42 2013 -0700
+++ b/Lib/multiprocessing/__init__.py	Mon Mar 18 11:57:00 2013 -0400
@@ -91,9 +91,10 @@
         except (ValueError, KeyError):
             num = 0
     elif 'bsd' in sys.platform or sys.platform == 'darwin':
-        comm = '/sbin/sysctl -n hw.ncpu'
         if sys.platform == 'darwin':
-            comm = '/usr' + comm
+            comm = '/usr/sbin/sysctl -n hw.activecpu'
+        else:
+            comm = '/sbin/sysctl -n hw.ncpu'
         try:
             with os.popen(comm) as p:
                 num = int(p.read())