diff --git a/numpy/lib/npyio.py b/numpy/lib/npyio.py
index dced7fd78649..d559ad5820d6 100644
--- a/numpy/lib/npyio.py
+++ b/numpy/lib/npyio.py
@@ -1032,7 +1032,13 @@ def savetxt(fname, X, fmt='%.18e', delimiter=' ', newline='\n', header='',
             if len(fmt) != ncol:
                 raise AttributeError('fmt has wrong shape.  %s' % str(fmt))
             format = asstr(delimiter).join(map(asstr, fmt))
-        elif isinstance(fmt, str):
+        elif isinstance(fmt, basestring):
+            if isinstance(fmt, unicode):
+                try:
+                    fmt = fmt.encode('ascii')
+                except UnicodeEncodeError:
+                    raise ValueError('invalid unicode in fmt: %r' % (fmt,))
+
             n_fmt_chars = fmt.count('%')
             error = ValueError('fmt has wrong number of %% formats:  %s' % fmt)
             if n_fmt_chars == 1:
diff --git a/numpy/lib/tests/test_io.py b/numpy/lib/tests/test_io.py
index 66310a509c43..1fff6adcfd89 100644
--- a/numpy/lib/tests/test_io.py
+++ b/numpy/lib/tests/test_io.py
@@ -15,7 +15,7 @@
 import numpy.ma as ma
 from numpy.lib._iotools import (ConverterError, ConverterLockError,
                                 ConversionWarning)
-from numpy.compat import asbytes, asbytes_nested, bytes, asstr
+from numpy.compat import asbytes, asbytes_nested, bytes, asstr, sixu
 from nose import SkipTest
 from numpy.ma.testutils import (TestCase, assert_equal, assert_array_equal,
                                 assert_raises, run_module_suite)
@@ -330,6 +330,19 @@ def test_format(self):
         # Bad fmt, should raise a ValueError
         c = BytesIO()
         assert_raises(ValueError, np.savetxt, c, a, fmt=99)
+        
+        # Unicode fmt is casted if possible
+        c1 = BytesIO()
+        c2 = BytesIO()
+        np.savetxt(c1, a, fmt='%02d : %3.1f')
+        np.savetxt(c2, a, fmt=sixu('%02d : %3.1f'))
+        c1.seek(0)
+        c2.seek(0)
+        assert_equal(c1.readlines(), c2.readlines())
+
+        # ... but raises an error if not.
+        c = BytesIO()
+        assert_raises(ValueError, np.savetxt(c, a, fmt=sixu('\xe7')))
 
     def test_header_footer(self):
         """