diff --git a/data/test/TreeBidiMap.emptyCollection.version3.3.obj b/data/test/TreeBidiMap.emptyCollection.version3.3.obj
new file mode 100644
index 000000000..d74791e35
Binary files /dev/null and b/data/test/TreeBidiMap.emptyCollection.version3.3.obj differ
diff --git a/data/test/TreeBidiMap.fullCollection.version3.3.obj b/data/test/TreeBidiMap.fullCollection.version3.3.obj
new file mode 100644
index 000000000..492cdc58e
Binary files /dev/null and b/data/test/TreeBidiMap.fullCollection.version3.3.obj differ
diff --git a/src/java/org/apache/commons/collections/bidimap/TreeBidiMap.java b/src/java/org/apache/commons/collections/bidimap/TreeBidiMap.java
index ad08d6b7f..efeeec3bf 100644
--- a/src/java/org/apache/commons/collections/bidimap/TreeBidiMap.java
+++ b/src/java/org/apache/commons/collections/bidimap/TreeBidiMap.java
@@ -16,6 +16,8 @@
  */
 package org.apache.commons.collections.bidimap;
 
+import java.io.Serializable;
+
 import java.util.AbstractSet;
 import java.util.Collection;
 import java.util.ConcurrentModificationException;
@@ -73,7 +75,7 @@ import org.apache.commons.collections.keyvalue.UnmodifiableMapEntry;
  * @author Marc Johnson
  * @author Stephen Colebourne
  */
-public class TreeBidiMap implements OrderedBidiMap {
+public class TreeBidiMap implements OrderedBidiMap, Serializable {
 
     private static final int KEY = 0;
     private static final int VALUE = 1;
@@ -87,10 +89,11 @@ public class TreeBidiMap implements OrderedBidiMap {
     private Node[] rootNode = new Node[2];
     private int nodeCount = 0;
     private int modifications = 0;
-    private Set keySet;
-    private Set valuesSet;
-    private Set entrySet;
-    private TreeBidiMap.Inverse inverse = null;
+
+    private transient Set keySet;
+    private transient Set valuesSet;
+    private transient Set entrySet;
+    private transient TreeBidiMap.Inverse inverse = null;
 
     //-----------------------------------------------------------------------
     /**
@@ -148,7 +151,7 @@ public class TreeBidiMap implements OrderedBidiMap {
     }
 
     /**
-     * Checks whether this map contains the a mapping for the specified value.
+     * Checks whether this map contains a mapping for the specified value.
      * <p>
      * The value must implement <code>Comparable</code>.
      *
@@ -1707,7 +1710,7 @@ public class TreeBidiMap implements OrderedBidiMap {
     /**
      * A node used to store the data.
      */
-    static class Node implements Map.Entry, KeyValue {
+    static class Node implements Map.Entry, KeyValue, Serializable {
 
         private Comparable[] data;
         private Node[] leftNode;
diff --git a/src/test/org/apache/commons/collections/bidimap/TestTreeBidiMap.java b/src/test/org/apache/commons/collections/bidimap/TestTreeBidiMap.java
index 9200f7798..22093c40a 100644
--- a/src/test/org/apache/commons/collections/bidimap/TestTreeBidiMap.java
+++ b/src/test/org/apache/commons/collections/bidimap/TestTreeBidiMap.java
@@ -73,4 +73,15 @@ public class TestTreeBidiMap extends AbstractTestOrderedBidiMap {
         return false;
     }
     
+    public String getCompatibilityVersion() {
+        return "3.3";
+    }
+
+//    public void testCreate() throws Exception {
+//        resetEmpty();
+//        writeExternalFormToDisk((java.io.Serializable) map, "/tmp/TreeBidiMap.emptyCollection.version3.3.obj");
+//        resetFull();
+//        writeExternalFormToDisk((java.io.Serializable) map, "/tmp/TreeBidiMap.fullCollection.version3.3.obj");
+//    }
+
 }