diff --git a/eo/src/EO.h b/eo/src/EO.h
index fb93d9aa..1fa1e94c 100644
--- a/eo/src/EO.h
+++ b/eo/src/EO.h
@@ -57,7 +57,7 @@ public:
   virtual ~EO() {};
 
   /// Return fitness value.
-  Fitness fitness() const {
+  const Fitness& fitness() const {
     if (invalid())
       throw std::runtime_error("invalid fitness");
     return repFitness;