diff --git a/src/cell.h b/src/cell.h
index 117eb708f2291426c5d546cd9363a4e42649cc87..0824f71d020e9063be86421febf1e9923ae44aff 100644
--- a/src/cell.h
+++ b/src/cell.h
@@ -269,7 +269,7 @@ struct cell {
   struct cell *super;
 
   /*! Cell flags bit-mask. */
-  uint32_t flags;
+  volatile uint32_t flags;
 
   /*! Hydro variables */
   struct {