diff --git a/src/java/com/threerings/miso/tile/BaseTileSet.java b/src/java/com/threerings/miso/tile/BaseTileSet.java
index 8c61b3145..f26acd303 100644
--- a/src/java/com/threerings/miso/tile/BaseTileSet.java
+++ b/src/java/com/threerings/miso/tile/BaseTileSet.java
@@ -1,5 +1,5 @@
 //
-// $Id: BaseTileSet.java,v 1.8 2001/11/29 00:17:32 mdb Exp $
+// $Id: BaseTileSet.java,v 1.9 2001/11/29 21:58:49 mdb Exp $
 
 package com.threerings.miso.tile;
 
@@ -27,6 +27,14 @@ public class BaseTileSet extends SwissArmyTileSet
         _passable = passable;
     }
 
+    /**
+     * Returns the passability information for the tiles in this tileset.
+     */
+    public boolean[] getPassability ()
+    {
+        return _passable;
+    }
+
     // documentation inherited
     protected Tile createTile (int tileIndex, Image image)
     {
diff --git a/src/java/com/threerings/miso/tile/tools/xml/BaseTileSetRuleSet.java b/src/java/com/threerings/miso/tile/tools/xml/BaseTileSetRuleSet.java
index bc11d2796..dbef0df56 100644
--- a/src/java/com/threerings/miso/tile/tools/xml/BaseTileSetRuleSet.java
+++ b/src/java/com/threerings/miso/tile/tools/xml/BaseTileSetRuleSet.java
@@ -1,5 +1,5 @@
 //
-// $Id: BaseTileSetRuleSet.java,v 1.6 2001/11/29 00:18:33 mdb Exp $
+// $Id: BaseTileSetRuleSet.java,v 1.7 2001/11/29 21:58:49 mdb Exp $
 
 package com.threerings.miso.tools.tile.xml;
 
@@ -11,6 +11,8 @@ import com.samskivert.xml.CallMethodSpecialRule;
 
 import com.threerings.media.tile.TileSet;
 import com.threerings.media.tools.tile.xml.SwissArmyTileSetRuleSet;
+
+import com.threerings.miso.Log;
 import com.threerings.miso.tile.BaseTileSet;
 
 /**
@@ -43,6 +45,22 @@ public class BaseTileSetRuleSet extends SwissArmyTileSetRuleSet
             });
     }
 
+    // documentation inherited
+    public boolean isValid (Object target)
+    {
+        BaseTileSet set = (BaseTileSet)target;
+        boolean valid = super.isValid(target);
+
+        // check for a <passable> element
+        if (set.getPassability() == null) {
+            Log.warning("Tile set definition missing valid <passable> " +
+                        "element [set=" + set + "].");
+            valid = false;
+        }
+
+        return valid;
+    }
+
     // documentation inherited
     protected Class getTileSetClass ()
     {