diff --git a/command.py b/command.py
index 2ee0a43..936ab40 100644
--- a/command.py
+++ b/command.py
@@ -15,6 +15,7 @@
 
 import os
 import optparse
+import platform
 import re
 import sys
 
@@ -69,7 +70,7 @@
 
     groups = mp.config.GetString('manifest.groups')
     if groups is None:
-      groups = 'default'
+      groups = 'default,platform-' + platform.system().lower()
     groups = [x for x in re.split('[,\s]+', groups) if x]
 
     if not args:
diff --git a/repo b/repo
index 0aabc0d..860a15d 100755
--- a/repo
+++ b/repo
@@ -129,6 +129,11 @@
                  dest='groups', default='default',
                  help='restrict manifest projects to ones with a specified group',
                  metavar='GROUP')
+group.add_option('-p', '--platform',
+                 dest='platform', default="auto",
+                 help='restrict manifest projects to ones with a specified'
+                      'platform group [auto|all|none|linux|darwin|...]',
+                 metavar='PLATFORM')
 
 
 # Tool
diff --git a/subcmds/init.py b/subcmds/init.py
index d1c497c..2e3215b 100644
--- a/subcmds/init.py
+++ b/subcmds/init.py
@@ -14,6 +14,7 @@
 # limitations under the License.
 
 import os
+import platform
 import re
 import shutil
 import sys
@@ -91,6 +92,11 @@
                  dest='groups', default='default',
                  help='restrict manifest projects to ones with a specified group',
                  metavar='GROUP')
+    g.add_option('-p', '--platform',
+                 dest='platform', default='auto',
+                 help='restrict manifest projects to ones with a specified'
+                      'platform group [auto|all|none|linux|darwin|...]',
+                 metavar='PLATFORM')
 
     # Tool
     g = p.add_option_group('repo Version options')
@@ -141,9 +147,24 @@
       r.Save()
 
     groups = re.split('[,\s]+', opt.groups)
+    all_platforms = ['linux', 'darwin']
+    platformize = lambda x: 'platform-' + x
+    if opt.platform == 'auto':
+      if (not opt.mirror and
+          not m.config.GetString('repo.mirror') == 'true'):
+        groups.append(platformize(platform.system().lower()))
+    elif opt.platform == 'all':
+      groups.extend(map(platformize, all_platfroms))
+    elif opt.platform in all_platforms:
+      groups.extend(platformize(opt.platform))
+    elif opt.platform != 'none':
+      print >>sys.stderr, 'fatal: invalid platform flag'
+      sys.exit(1)
+
     groups = [x for x in groups if x]
     groupstr = ','.join(groups)
-    if groupstr == 'default':
+    if opt.platform == 'auto' and
+       groupstr == 'default,platform-' + platform.system.lower():
       groupstr = None
     m.config.SetString('manifest.groups', groupstr)