diff --git a/gtsam/inference/VariableIndex.h b/gtsam/inference/VariableIndex.h
index f503516a4..570369e0a 100644
--- a/gtsam/inference/VariableIndex.h
+++ b/gtsam/inference/VariableIndex.h
@@ -161,6 +161,16 @@ protected:
   template<class FG> void fill(const FG& factorGraph);
 
   /// @}
+
+private:
+  /** Serialization function */
+  friend class boost::serialization::access;
+  template<class ARCHIVE>
+  void serialize(ARCHIVE & ar, const unsigned int version) {
+    ar & BOOST_SERIALIZATION_NVP(index_);
+    ar & BOOST_SERIALIZATION_NVP(nFactors_);
+    ar & BOOST_SERIALIZATION_NVP(nEntries_);
+  }
 };
 
 /* ************************************************************************* */