diff -r 385b871cffae Lib/locale.py
--- a/Lib/locale.py	Sat Feb 16 15:34:57 2008 +0100
+++ b/Lib/locale.py	Sat Feb 16 21:01:15 2008 +0100
@@ -278,6 +278,13 @@ def _test():
 # overridden below)
 _setlocale = setlocale
 
+# Avoid relying on the locale-dependent .lower() method
+# (see bug #1813).
+_ascii_lower_map = ''.join(
+    chr(x + 32 if x >= ord('A') and x <= ord('Z') else x)
+    for x in range(256)
+)
+
 def normalize(localename):
 
     """ Returns a normalized locale code for the given locale
@@ -295,7 +302,7 @@ def normalize(localename):
 
     """
     # Normalize the locale name and extract the encoding
-    fullname = localename.lower()
+    fullname = localename.translate(_ascii_lower_map)
     if ':' in fullname:
         # ':' is sometimes used as encoding delimiter.
         fullname = fullname.replace(':', '.')
diff -r 385b871cffae Lib/test/test_codecs.py
--- a/Lib/test/test_codecs.py	Sat Feb 16 15:34:57 2008 +0100
+++ b/Lib/test/test_codecs.py	Sat Feb 16 21:01:15 2008 +0100
@@ -1,6 +1,7 @@ from test import test_support
 from test import test_support
 import unittest
 import codecs
+import locale
 import sys, StringIO, _testcapi
 
 class Queue(object):
@@ -1078,6 +1079,16 @@ class CodecsModuleTest(unittest.TestCase
         self.assertRaises(LookupError, codecs.lookup, "__spam__")
         self.assertRaises(LookupError, codecs.lookup, " ")
 
+    def test_lookup_with_locale(self):
+        # Bug #1813: when normalizing codec name, lowercasing must be locale
+        # agnostic, otherwise the looked up codec name might end up wrong.
+        try:
+            locale.setlocale(locale.LC_CTYPE, 'tr')
+        except locale.Error:
+            # SKIPped test
+            return
+        codecs.lookup('ISO8859_1')
+
     def test_getencoder(self):
         self.assertRaises(TypeError, codecs.getencoder)
         self.assertRaises(LookupError, codecs.getencoder, "__spam__")
diff -r 385b871cffae Python/codecs.c
--- a/Python/codecs.c	Sat Feb 16 15:34:57 2008 +0100
+++ b/Python/codecs.c	Sat Feb 16 21:01:15 2008 +0100
@@ -45,6 +45,22 @@ int PyCodec_Register(PyObject *search_fu
     return -1;
 }
 
+/* A locale-agnostic version of tolower(), to avoid codec lookup failures
+   when the current locale's case folding behaviour differs from latin-based
+   languages (see bug #1813).
+   XXX: do we want to export this function and make it available to other
+   modules?
+   */
+
+static
+int ascii_tolower(int c)
+{
+    if (c >= 'A' && c <= 'Z')
+        return c + 32;
+    return c;
+}
+
+
 /* Convert a string to a normalized Python string: all characters are
    converted to lower case, spaces are replaced with underscores. */
 
@@ -70,7 +86,7 @@ PyObject *normalizestring(const char *st
         if (ch == ' ')
             ch = '-';
         else
-            ch = tolower(Py_CHARMASK(ch));
+            ch = ascii_tolower(Py_CHARMASK(ch));
 	p[i] = ch;
     }
     return v;