diff --git a/include/boost/fusion/support/detail/unknown_key.hpp b/include/boost/fusion/support/detail/unknown_key.hpp
deleted file mode 100644
index 9466b9c0..00000000
--- a/include/boost/fusion/support/detail/unknown_key.hpp
+++ /dev/null
@@ -1,16 +0,0 @@
-/*=============================================================================
-    Copyright (c) 2001-2011 Joel de Guzman
-
-    Distributed under the Boost Software License, Version 1.0. (See accompanying 
-    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
-==============================================================================*/
-#if !defined(FUSION_UNKNOWN_KEY_09242005_1219)
-#define FUSION_UNKNOWN_KEY_09242005_1219
-
-namespace boost { namespace fusion { namespace detail
-{
-    template <int index>
-    struct unknown_key {};
-}}}
-
-#endif