Index: _test_real_ftp.py
===================================================================
--- _test_real_ftp.py	(revision 717)
+++ _test_real_ftp.py	(working copy)
@@ -173,6 +173,15 @@
         host.chdir(host.pardir)
         host.rmdir('/_dir1_')
 
+    def test_makedirs_from_non_root_directory_fake_windows_os(self):
+        saved_sep = os.sep
+        os.sep = '\\'
+        try:
+            self.test_makedirs_from_non_root_directory()
+        except:
+            os.sep = saved_sep
+            raise
+
     def test_makedirs_of_existing_directory(self):
         host = self.host
         # the (chrooted) login directory
Index: ftputil.py
===================================================================
--- ftputil.py	(revision 717)
+++ ftputil.py	(working copy)
@@ -609,7 +609,7 @@
         #  the "lowermost" directory
         for index in range(1, len(directories)):
             # re-insert the separator which got lost by using `path.split`
-            next_directory = os.sep + self.path.join(*directories[:index+1])
+            next_directory = self.sep + self.path.join(*directories[:index+1])
             try:
                 self.mkdir(next_directory)
             except ftp_error.PermanentError: