Differences in yield and combining ability of 5 synthetic strains of maize, all developed from the same parent variety, were studied. The strains included the parent variety; 1 strain developed by intercrossing 10 elite lines selected from the parent variety; and 3 strains which were developed by different recurrent selection programs. The 5 strains, all possible crosses among strains, and crosses of the 5 strains to 4 testers were grown in field trials in 13 environments. All the selected strains were higher yielding than the parent variety. One of the selected strains outyielded the parent variety by 22%. The mean yield of the crosses among strains exceeded the mean yield of the strains per se by 11%. Testcross yields indicated that progress from selection in the 3 strains developed by recurrent selection ranged from approximately 1.1% to 2.5% per cycle of selection. Considerable genetic divergence among strains occurred.