diff --git a/tracker/rabit_hadoop.py b/tracker/rabit_hadoop.py
index 4a2cdb718..79e0cb133 100755
--- a/tracker/rabit_hadoop.py
+++ b/tracker/rabit_hadoop.py
@@ -35,7 +35,7 @@ if hadoop_streaming_jar != None:
   args.hadoop_streaming_jar = hadoop_streaming_jar
 
 def hadoop_streaming(nslaves, slave_args):
-  cmd = '%s jar %s -input %s -output %s -mapper \"%s stdin %d %d stdout %s\" -reducer \"/bin/cat\" -file %s' % (args.hadoop_binary, args.hadoop_streaming_jar, args.input, args.output, args.mapper, args.nclusters, args.iterations, ' '.join(slave_args), args.mapper)
+  cmd = '%s jar %s -input %s -output %s -mapper \"%s stdin %d %d stdout %s\" -reducer \"/bin/cat\" -file %s -D mapred.map.tasks=%d' % (args.hadoop_binary, args.hadoop_streaming_jar, args.input, args.output, args.mapper, args.nclusters, args.iterations, ' '.join(slave_args), args.mapper, nslaves)
   print cmd
   subprocess.check_call(cmd, shell = True)