diff --git a/gcloud/datastore/connection.py b/gcloud/datastore/connection.py
index 13e299a16f93..cd04302496bd 100644
--- a/gcloud/datastore/connection.py
+++ b/gcloud/datastore/connection.py
@@ -165,23 +165,20 @@ def begin_transaction(self, dataset_id, serializable=False):
 
     return response.transaction
 
-  def rollback_transaction(self, dataset_id, transaction_id):
-    """Rollback an existing transaction.
+  def rollback_transaction(self, dataset_id):
+    """Rollback the connection's existing transaction.
 
     Raises a ``ValueError``
     if the connection isn't currently in a transaction.
 
     :type dataset_id: string
     :param dataset_id: The dataset to which the transaction belongs.
-
-    :type transaction_id: string
-    :param transaction_id: The ID of the transaction to roll back.
     """
     if not self.transaction() or not self.transaction().id():
       raise ValueError('No transaction to rollback.')
 
     request = datastore_pb.RollbackRequest()
-    request.transaction = transaction_id
+    request.transaction = self.transaction().id()
     # Nothing to do with this response, so just execute the method.
     self._rpc(dataset_id, 'rollback', request,
               datastore_pb.RollbackResponse)
diff --git a/gcloud/datastore/test_connection.py b/gcloud/datastore/test_connection.py
index f420af347bb4..ee151c261d0b 100644
--- a/gcloud/datastore/test_connection.py
+++ b/gcloud/datastore/test_connection.py
@@ -244,31 +244,27 @@ def test_begin_transaction_explicit_serialize(self):
 
     def test_rollback_transaction_wo_existing_transaction(self):
         DATASET_ID = 'DATASET'
-        TRANSACTION_ID = 'TRANSACTION'
         conn = self._makeOne()
         self.assertRaises(ValueError,
-                          conn.rollback_transaction, DATASET_ID, TRANSACTION_ID)
+                          conn.rollback_transaction, DATASET_ID)
 
     def test_rollback_transaction_w_existing_transaction_no_id(self):
         class Xact(object):
             def id(self):
                 return None
         DATASET_ID = 'DATASET'
-        TRANSACTION_ID = 'TRANSACTION'
         conn = self._makeOne()
         conn.transaction(Xact())
         self.assertRaises(ValueError,
-                          conn.rollback_transaction, DATASET_ID, TRANSACTION_ID)
+                          conn.rollback_transaction, DATASET_ID)
 
     def test_rollback_transaction_ok(self):
         from gcloud.datastore.connection import datastore_pb
+        DATASET_ID = 'DATASET'
+        TRANSACTION = 'xact'
         class Xact(object):
             def id(self):
-                return 'xact'
-        xact = object()
-
-        DATASET_ID = 'DATASET'
-        TRANSACTION = 'TRANSACTION'
+                return TRANSACTION
         rsp_pb = datastore_pb.RollbackResponse()
         conn = self._makeOne()
         conn.transaction(Xact())
@@ -280,14 +276,13 @@ def id(self):
                         'rollback',
                        ])
         http = conn._http = Http({'status': '200'}, rsp_pb.SerializeToString())
-        self.assertEqual(conn.rollback_transaction(DATASET_ID, TRANSACTION),
-                         None)
+        self.assertEqual(conn.rollback_transaction(DATASET_ID), None)
         cw = http._called_with
         self.assertEqual(cw['uri'], URI)
         self.assertEqual(cw['method'], 'POST')
         self.assertEqual(cw['headers'],
                             {'Content-Type': 'application/x-protobuf',
-                             'Content-Length': '13',
+                             'Content-Length': '6',
                             })
         rq_class = datastore_pb.RollbackRequest
         request = rq_class()