diff --git a/requests/compat.py b/requests/compat.py
index 8e4a17e4..af0207de 100644
--- a/requests/compat.py
+++ b/requests/compat.py
@@ -91,6 +91,7 @@ if is_py2:
     from StringIO import StringIO
     from .packages import chardet
 
+    builtin_str = str
     bytes = str
     str = unicode
     basestring = basestring
@@ -105,6 +106,7 @@ elif is_py3:
     from io import StringIO
     from .packages import chardet2 as chardet
 
+    builtin_str = str
     str = str
     bytes = bytes
     basestring = (str,bytes)
diff --git a/requests/models.py b/requests/models.py
index 9b248d31..514bcb40 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -32,7 +32,7 @@ from .utils import (
     DEFAULT_CA_BUNDLE_PATH)
 from .compat import (
     cookielib, urlparse, urlunparse, urljoin, urlsplit, urlencode, str, bytes,
-    StringIO, is_py2, chardet, json)
+    StringIO, is_py2, chardet, json, builtin_str)
 
 REDIRECT_STATI = (codes.moved, codes.found, codes.other, codes.temporary_moved)
 CONTENT_CHUNK_SIZE = 10 * 1024
@@ -507,7 +507,7 @@ class Request(object):
             if self.data:
 
                 body = self._encode_params(self.data)
-                if isinstance(self.data, str) or hasattr(self.data, 'read'):
+                if isinstance(self.data, str) or isinstance(self.data, builtin_str) or hasattr(self.data, 'read'):
                     content_type = None
                 else:
                     content_type = 'application/x-www-form-urlencoded'
diff --git a/tests/test_requests.py b/tests/test_requests.py
index 076ca6ce..6fe0d758 100755
--- a/tests/test_requests.py
+++ b/tests/test_requests.py
@@ -1029,5 +1029,12 @@ class RequestsTestSuite(TestSetup, TestBaseMixin, unittest.TestCase):
         self.assertEqual(t.get('form'), {'field': 'a, b'})
         self.assertEqual(t.get('files'), files)
 
+    def test_str_data_content_type(self):
+        data = 'test string data'
+        r = post(httpbin('post'), data=data)
+        t = json.loads(r.text)
+        self.assertEqual(t.get('headers').get('Content-Type'), '')
+
+
 if __name__ == '__main__':
     unittest.main()