diff --git a/lib/OpenLayers/Layer/WMS.js b/lib/OpenLayers/Layer/WMS.js
index 9a34533148..c4b3dd571d 100644
--- a/lib/OpenLayers/Layer/WMS.js
+++ b/lib/OpenLayers/Layer/WMS.js
@@ -45,9 +45,12 @@ OpenLayers.Layer.WMS.prototype =
                            );
         }
 
-        // if the layer is transparent, it will be an overlay        
-        this.isBaseLayer = ((this.params.TRANSPARENT != "true") && 
-                            (this.params.TRANSPARENT != true));
+        // unless explicitly set in options, if the layer is transparent, 
+        // it will be an overlay        
+        if ((options == null) || (options.isBaseLayer == null)) {
+            this.isBaseLayer = ((this.params.TRANSPARENT != "true") && 
+                                (this.params.TRANSPARENT != true));
+        }
     },    
 
     /**
diff --git a/lib/OpenLayers/Layer/WMS/Untiled.js b/lib/OpenLayers/Layer/WMS/Untiled.js
index afb875d546..bb0f211f7d 100644
--- a/lib/OpenLayers/Layer/WMS/Untiled.js
+++ b/lib/OpenLayers/Layer/WMS/Untiled.js
@@ -50,10 +50,12 @@ OpenLayers.Layer.WMS.Untiled.prototype =
                            );
         }
 
-        // if the layer is transparent, it will be an overlay        
-        this.isBaseLayer = ((this.params.TRANSPARENT != "true") && 
-                            (this.params.TRANSPARENT != true));
-
+        // unless explicitly set in options, if the layer is transparent, 
+        // it will be an overlay        
+        if ((options == null) ||(options.isBaseLayer == null)) {
+            this.isBaseLayer = ((this.params.TRANSPARENT != "true") && 
+                                (this.params.TRANSPARENT != true));
+        }
     },    
 
     /**