diff --git a/src/micropolisj/engine/Bulldozer.java b/src/micropolisj/engine/Bulldozer.java
index 8194033..7700031 100644
--- a/src/micropolisj/engine/Bulldozer.java
+++ b/src/micropolisj/engine/Bulldozer.java
@@ -26,10 +26,10 @@ class Bulldozer extends ToolStroke
 		for (int y = 0; y < b.height; y++) {
 			for (int x = 0; x < b.width; x++) {
 
-				int tile = eff.getTile(b.x+x, b.y+y);
-				if (isDozeable(tile) && !isZoneCenter(tile)) {
+				ToolEffectIfc subEff = new TranslatedToolEffect(eff, b.x+x, b.y+y);
+				if (city.isTileDozeable(subEff)) {
 
-					dozeField(new TranslatedToolEffect(eff, b.x+x, b.y+y));
+					dozeField(subEff);
 				}
 
 			}
@@ -81,9 +81,6 @@ class Bulldozer extends ToolStroke
 	{
 		int tile = eff.getTile(0, 0);
 
-		// check dozeable bit
-		assert isDozeable(tile);
-
 		if (isOverWater(tile))
 		{
 			// dozing over water, replace with water.
diff --git a/src/micropolisj/engine/Micropolis.java b/src/micropolisj/engine/Micropolis.java
index 3a3530a..9c13fd5 100644
--- a/src/micropolisj/engine/Micropolis.java
+++ b/src/micropolisj/engine/Micropolis.java
@@ -428,6 +428,32 @@ public class Micropolis
 		return map[ypos][xpos];
 	}
 
+	boolean isTileDozeable(ToolEffectIfc eff)
+	{
+		int myTile = eff.getTile(0, 0) & LOMASK;
+		TileSpec ts = Tiles.get(myTile);
+		if (ts.canBulldoze) {
+			return true;
+		}
+
+		if (ts.owner != null) {
+			// part of a zone; only bulldozeable if the owner tile is
+			// no longer intact.
+
+			int baseTile = eff.getTile(-ts.ownerOffsetX, -ts.ownerOffsetY) & LOMASK;
+			return !(ts.owner.tileNumber == baseTile);
+		}
+
+		return false;
+	}
+
+	boolean isTileDozeable(int xpos, int ypos)
+	{
+		return isTileDozeable(
+			new ToolEffect(this, xpos, ypos)
+			);
+	}
+
 	public boolean isTilePowered(int xpos, int ypos)
 	{
 		return (getTile(xpos, ypos) & PWRBIT) == PWRBIT;
@@ -2018,7 +2044,7 @@ public class Micropolis
 				if (isCombustible(z)) {
 					z |= 8192;   //synthesize BURNBIT on export
 				}
-				if (isDozeable(z)) {
+				if (isTileDozeable(x, y)) {
 					z |= 4096;   //synthesize BULLBIT on export
 				}
 				if (isAnimated(z)) {