diff --git a/boost/iostreams/filter/symmetric.hpp b/boost/iostreams/filter/symmetric.hpp
index cc92b0c..b8864e0 100644
--- a/boost/iostreams/filter/symmetric.hpp
+++ b/boost/iostreams/filter/symmetric.hpp
@@ -185,6 +185,7 @@ public:
         }
     }
     SymmetricFilter& filter() { return *pimpl_; }
+    const SymmetricFilter& filter() const { return *pimpl_; }
     string_type unconsumed_input() const;
 
 // Give impl access to buffer_type on Tru64