diff --git a/core/src/main/scala/spark/PairRDDFunctions.scala b/core/src/main/scala/spark/PairRDDFunctions.scala
index cc3cca2571..112beb2320 100644
--- a/core/src/main/scala/spark/PairRDDFunctions.scala
+++ b/core/src/main/scala/spark/PairRDDFunctions.scala
@@ -62,7 +62,9 @@ class PairRDDFunctions[K: ClassManifest, V: ClassManifest](
     }
     val aggregator =
       new Aggregator[K, V, C](createCombiner, mergeValue, mergeCombiners)
-    if (mapSideCombine) {
+    if (self.partitioner == Some(partitioner)) {
+      self.mapPartitions(aggregator.combineValuesByKey(_), true)
+    } else if (mapSideCombine) {
       val mapSideCombined = self.mapPartitions(aggregator.combineValuesByKey(_), true)
       val partitioned = new ShuffledRDD[K, C](mapSideCombined, partitioner)
       partitioned.mapPartitions(aggregator.combineCombinersByKey(_), true)
diff --git a/core/src/test/scala/spark/ShuffleSuite.scala b/core/src/test/scala/spark/ShuffleSuite.scala
index 3493b9511f..50f2b294bf 100644
--- a/core/src/test/scala/spark/ShuffleSuite.scala
+++ b/core/src/test/scala/spark/ShuffleSuite.scala
@@ -1,6 +1,7 @@
 package spark
 
 import scala.collection.mutable.ArrayBuffer
+import scala.collection.mutable.HashSet
 
 import org.scalatest.FunSuite
 import org.scalatest.matchers.ShouldMatchers
@@ -98,6 +99,28 @@ class ShuffleSuite extends FunSuite with ShouldMatchers with LocalSparkContext {
     val sums = pairs.reduceByKey(_+_, 10).collect()
     assert(sums.toSet === Set((1, 7), (2, 1)))
   }
+  
+  test("reduceByKey with partitioner") {
+    sc = new SparkContext("local", "test")
+    val p = new Partitioner() {
+      def numPartitions = 2
+      def getPartition(key: Any) = key.asInstanceOf[Int]
+    }
+    val pairs = sc.parallelize(Array((1, 1), (1, 2), (1, 1), (0, 1))).partitionBy(p)
+    val sums = pairs.reduceByKey(_+_)
+    assert(sums.collect().toSet === Set((1, 4), (0, 1)))
+    assert(sums.partitioner === Some(p))
+    // count the dependencies to make sure there is only 1 ShuffledRDD
+    val deps = new HashSet[RDD[_]]()
+    def visit(r: RDD[_]) {
+      for (dep <- r.dependencies) {
+        deps += dep.rdd
+        visit(dep.rdd)
+      }
+    }
+    visit(sums)
+    assert(deps.size === 2) // ShuffledRDD, ParallelCollection
+  }
 
   test("join") {
     sc = new SparkContext("local", "test")