diff --git a/doc/hgrc.5.txt b/doc/hgrc.5.txt
--- a/doc/hgrc.5.txt
+++ b/doc/hgrc.5.txt
@@ -556,6 +556,12 @@ Supported arguments:
   tool. Mercurial will search for this key first under
   ``HKEY_CURRENT_USER`` and then under ``HKEY_LOCAL_MACHINE``.
   Default: None
+``regkeyalt``
+  An alternate Windows registry key to try if the first key is not
+  found.  The alternate key uses the same ``regname`` and ``regappend``
+  semantics of the primary key.  The most common use for this key
+  is to search for 32bit applications on 64bit operating systems.
+  Default: None
 ``regname``
   Name of value to read from specified registry key. Defaults to the
   unnamed (default) value.
diff --git a/mercurial/filemerge.py b/mercurial/filemerge.py
--- a/mercurial/filemerge.py
+++ b/mercurial/filemerge.py
@@ -25,8 +25,10 @@ def _toollist(ui, tool, part, default=[]
 def _findtool(ui, tool):
     if tool in _internal:
         return tool
-    k = _toolstr(ui, tool, "regkey")
-    if k:
+    for kn in ("regkey", "regkeyalt"):
+        k = _toolstr(ui, tool, kn)
+        if not k:
+            continue
         p = util.lookup_reg(k, _toolstr(ui, tool, "regname"))
         if p:
             p = util.find_exe(p + _toolstr(ui, tool, "regappend"))