diff --git a/dataset/freeze/format/fcsv.py b/dataset/freeze/format/fcsv.py
index e246904..0a69920 100644
--- a/dataset/freeze/format/fcsv.py
+++ b/dataset/freeze/format/fcsv.py
@@ -47,4 +47,5 @@ class CSVSerializer(Serializer):
 
     def close(self):
         for writer, fh in self.handles.values():
-            fh.close()
+            if fh != self.fileobj:
+                fh.close()
diff --git a/dataset/freeze/format/fjson.py b/dataset/freeze/format/fjson.py
index 11a7714..b5783c3 100644
--- a/dataset/freeze/format/fjson.py
+++ b/dataset/freeze/format/fjson.py
@@ -55,4 +55,5 @@ class JSONSerializer(Serializer):
                                           self.export.get('callback'),
                                           data)
             fh.write(data)
-            fh.close()
+            if self.fileobj is None:
+                fh.close()
diff --git a/test/test_freeze.py b/test/test_freeze.py
index 09c8b98..82e6e07 100644
--- a/test/test_freeze.py
+++ b/test/test_freeze.py
@@ -62,6 +62,15 @@ class FreezeTestCase(unittest.TestCase):
         finally:
             fh.close()
 
+    def test_memory_streams(self):
+        import io
+
+        for fmt in ('csv', 'json', 'tabson'):
+            with io.StringIO() as fd:
+                freeze(self.tbl.all(), format=fmt, fileobj=fd)
+                self.assertFalse(fd.closed, 'fileobj was closed for format %s' % fmt)
+                fd.getvalue() # should not throw
+
 
 class SerializerTestCase(unittest.TestCase):