diff --git a/src/mtype.h b/src/mtype.h
index 2f1eb36896c4a..362312a862181 100644
--- a/src/mtype.h
+++ b/src/mtype.h
@@ -105,6 +105,7 @@ enum m_flag : int {
     MF_ELECTRIC,            // Shocks unarmed attackers
     MF_ACIDPROOF,           // Immune to acid
     MF_ACIDTRAIL,           // Leaves a trail of acid
+    MF_SHORTACIDTRAIL,           // Leaves an intermittent trail of acid
     MF_FIREPROOF,           // Immune to fire
     MF_SLUDGEPROOF,         // Ignores the effect of sludge trails
     MF_SLUDGETRAIL,         // Causes monster to leave a sludge trap trail when moving