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The effect of three vigor-control apple (Malus domestica Borkh.) rootstocks (seedling, MM.106, and M.7a) on fruit diameter of three cultivars (’Red Delicious’, ’Granny Smith’, and ’Gala’) was studied over two growing seasons (1990-91) in the arid climate of Willcox, Ariz. Daily fruit growth rate (DFGR) and effective fruit growth period (EFGP) data indicate cultivar differences in DFGR as well as EFGP. Cultivars with a high DFGR had a relatively shorter EFGP. Rootstocks had no significant effect on EFGP. Cultivar x rootstock interaction on fruit diameter was significant for DFGR, but not for EFGP. ’Red Delicious’ and ’Granny Smith’ trees produced larger fruits on MM.106 and M.7a than on seedling rootstock. For ’Gala’, there was no significant effect of all rootstocks on fruit diameter.