diff --git a/graphene/types/datetime.py b/graphene/types/datetime.py
index ca96547f..3519d76d 100644
--- a/graphene/types/datetime.py
+++ b/graphene/types/datetime.py
@@ -4,6 +4,7 @@ import datetime
 
 from aniso8601 import parse_date, parse_datetime, parse_time
 from graphql.language import ast
+from six import string_types
 
 from .scalars import Scalar
 
@@ -34,7 +35,7 @@ class Date(Scalar):
         try:
             if isinstance(value, datetime.date):
                 return value
-            elif isinstance(value, str):
+            elif isinstance(value, string_types):
                 return parse_date(value)
         except ValueError:
             return None
@@ -64,7 +65,7 @@ class DateTime(Scalar):
         try:
             if isinstance(value, datetime.datetime):
                 return value
-            elif isinstance(value, str):
+            elif isinstance(value, string_types):
                 return parse_datetime(value)
         except ValueError:
             return None
@@ -94,7 +95,7 @@ class Time(Scalar):
         try:
             if isinstance(value, datetime.time):
                 return value
-            elif isinstance(value, str):
+            elif isinstance(value, string_types):
                 return parse_time(value)
         except ValueError:
             return None