I believe you can simplify it significantly. Try this:

SELECT Municipalities.Municipality, Municipalities.MUNCode
FROM Municipalities
WHERE Municipalities.CTYCode = forms![thenameoftheformyouare on]![thenameofyourcitycodefield];


There is really no need for the join to the city table for the
condition is simply that the municipalities are part of the city
chosen in your prior combo.

Ron