diff --git a/doc/fixes37.0 b/doc/fixes37.0
index b2d17af47..8b766ebb2 100644
--- a/doc/fixes37.0
+++ b/doc/fixes37.0
@@ -630,6 +630,10 @@ obj->o_id might be set to invalid value 0 when a partly used up stack had a
 	dummy copy added to a shop's bill or when a bones file was loaded
 	(in theory that could happen on any system but in practice it could
 	only happen on a configuration that uses 16-bit ints)
+if a Rider or displacer beast swapped places with a single-segment long worm
+	the segment co-located with the head wasn't moved with that head;
+	if sanity_checking was enabled a warning could be triggered:
+	mon (000000) at seg location is not worm (123abc)
 
 
 Fixes to 3.7.0-x Problems that Were Exposed Via git Repository
diff --git a/src/mhitm.c b/src/mhitm.c
index 4f09298f9..11787acad 100644
--- a/src/mhitm.c
+++ b/src/mhitm.c
@@ -243,13 +243,22 @@ mdisplacem(register struct monst *magr, register struct monst *mdef,
     }
 
     remove_monster(fx, fy); /* pick up from orig position */
-    remove_monster(tx, ty);
+    if (mdef->wormno)
+        remove_worm(mdef);
+    else
+        remove_monster(tx, ty);
     place_monster(magr, tx, ty); /* put down at target spot */
     place_monster(mdef, fx, fy);
+    if (mdef->wormno) /* now put down tail */
+        place_worm_tail_randomly(mdef, fx, fy);
+    /* either creature might move into or out of a poison gas cloud */
+    update_monster_region(magr);
+    update_monster_region(mdef);
+
     if (g.vis && !quietly)
         pline("%s moves %s out of %s way!", Monnam(magr), mon_nam(mdef),
               is_rider(pa) ? "the" : mhis(magr));
-    newsym(fx, fy);  /* see it */
+    newsym(fx, fy);  /* see it       */
     newsym(tx, ty);  /*   all happen */
     flush_screen(0); /* make sure it shows up */