diff --git a/mercurial/ui.py b/mercurial/ui.py
--- a/mercurial/ui.py
+++ b/mercurial/ui.py
@@ -80,8 +80,8 @@ class ui(object):
             self.warn(_("Ignored: %s\n") % str(inst))
 
         if self.plain():
-            for k in ('debug', 'fallbackencoding', 'quiet', 'traceback',
-                      'verbose'):
+            for k in ('debug', 'fallbackencoding', 'quiet', 'slash',
+                      'traceback', 'verbose'):
                 if k in cfg['ui']:
                     del cfg['ui'][k]
             for k, v in cfg.items('alias'):
diff --git a/tests/test-hgrc b/tests/test-hgrc
--- a/tests/test-hgrc
+++ b/tests/test-hgrc
@@ -33,6 +33,7 @@ echo "[ui]" > $HGRCPATH
 echo "debug=true" >> $HGRCPATH
 echo "fallbackencoding=ASCII" >> $HGRCPATH
 echo "quiet=true" >> $HGRCPATH
+echo "slash=true" >> $HGRCPATH
 echo "traceback=true" >> $HGRCPATH
 echo "verbose=true" >> $HGRCPATH
 echo "[defaults]" >> $HGRCPATH
diff --git a/tests/test-hgrc.out b/tests/test-hgrc.out
--- a/tests/test-hgrc.out
+++ b/tests/test-hgrc.out
@@ -11,13 +11,14 @@ foo.bar=a\nb\nc\nde\nfg
 foo.baz=bif cb
 hg: config error at $HGRCPATH:1: cannot include /no-such-file (No such file or directory)
 % customized hgrc
-.../.hgrc:10: alias.log=log -g
-.../.hgrc:8: defaults.identify=-n
+.../.hgrc:11: alias.log=log -g
+.../.hgrc:9: defaults.identify=-n
 .../.hgrc:2: ui.debug=true
 .../.hgrc:3: ui.fallbackencoding=ASCII
 .../.hgrc:4: ui.quiet=true
-.../.hgrc:5: ui.traceback=true
-.../.hgrc:6: ui.verbose=true
+.../.hgrc:5: ui.slash=true
+.../.hgrc:6: ui.traceback=true
+.../.hgrc:7: ui.verbose=true
 % plain hgrc
 none: ui.traceback=True
 none: ui.verbose=False