diff --git a/src/hydro/Gadget2/hydro_part.h b/src/hydro/Gadget2/hydro_part.h
index 111e9d75dcff45480ff9374e4c49e2b644503c89..724cb23a546920714d2a2e4a4b1510220229ed7f 100644
--- a/src/hydro/Gadget2/hydro_part.h
+++ b/src/hydro/Gadget2/hydro_part.h
@@ -33,7 +33,9 @@
 
 #include "cooling_struct.h"
 
+#ifdef DEBUG_INTERACTIONS_SPH
 #define NUM_OF_NEIGHBOURS 128
+#endif
 
 /* Extra particle data not needed during the SPH loops over neighbours. */
 struct xpart {