The response to selection for perithecial number in Cochliobolus heterostrophus, a heterothallic ascomycete, is described. Thirteen MATA isolates and 12 MATa isolates of the fungus were mated in all possible combinations and evaluated for fertility. Progeny from some of these matings segregated for a defective perithecial allele. Selection for increased numbers of mature perithecia was carried out for six generations of intermatings among four to six parents in each generation which were selected from approximately 24 progeny from the preceeding generation. After six generations the numbers of perithecia had increased by nearly a factor of four. The proportion of additive genetic variance decreased linearly in the selection generations. The increase in perithecia was confirmed in concurrent mating tests among subpopulations from each selection generation.