Access – combo box

I am creating a database which has two fields among many. One field is Town and the second field is Zip. I have a table of certain town zip combinations the ky to which is town. I want ot be able to enter the town name via a comb box drop down and have the corisponding zip be filled in. Corspondingly I want to be able to fill in the zip and have the coresponding town be filled in I can get the town to be filled in but the zip will not fill in what Am I doing wrong.