diff --git a/Lib/imaplib.py b/Lib/imaplib.py
--- a/Lib/imaplib.py
+++ b/Lib/imaplib.py
@@ -509,6 +509,8 @@
 
         NB: 'password' will be quoted.
         """
+        if isinstance(password, str):
+            password = bytes(password, "ASCII")
         typ, dat = self._simple_command('LOGIN', user, self._quote(password))
         if typ != 'OK':
             raise self.error(dat[-1])