Index: Lib/test/test_io.py
===================================================================
--- Lib/test/test_io.py	(revision 57506)
+++ Lib/test/test_io.py	(working copy)
@@ -189,6 +189,12 @@
         f = io.BytesIO(data)
         self.read_ops(f, True)
 
+    def test_stringio_getvalue(self):
+        f = io.StringIO()
+        data = "Hello world\n\x82"
+        f.write(data)
+        self.assertEqual(data, f.getvalue())
+
     def test_large_file_ops(self):
         # On Windows and Mac OSX this test comsumes large resources; It takes
         # a long time to build the >2GB file and takes >2GB of disk space
Index: Lib/io.py
===================================================================
--- Lib/io.py	(revision 57506)
+++ Lib/io.py	(working copy)
@@ -1370,4 +1370,5 @@
 
     def getvalue(self):
         self.flush()
-        return self.buffer.getvalue().decode(self._encoding)
+        value = self.buffer.getvalue().decode(self._encoding)
+        return self._replacenl(value)