Cochliobolus heterostrophus strains near-isogenic except for a gene for toxin production were examined for differences in fitness on normal cytoplasm maize in the field and greenhouse. A mixture of two strains (one race O, non-toxin-producing, and one race T, toxin-producing) was used to inoculate a field plot of Cornell 281 in Ames, IA in 1983 and 1984. Different pairs of strains were used each year. The frequency of the race T strain decreased significantly (47% to 20% over 9 weeks in 1983 and 24% to 15% over 8 weeks in 1984) compared with the race O strain in both field seasons, showing that the race T strains were less fit than the race O strains. Differences in fitness of near-isogenic strains were also expressed as differences in lesion length. Lesions produced by race T lines were significantly shorter (an average of 28% in the greenhouse and 25% in the field) than those produced by race O lines. This was true for strains from four backcross generations of the fungus and on three genotypes of maize. No significant differences, however, were observed when infection efficiency and spore germinability of race T and race O strains were compared. Additional studies showed that the laboratory strains were less fit than naturally-occurring field strains. I concluded that the toxin gene or a gene closely linked to it reduces pathogenic fitness. Debilitation associated with the toxin locus could explain the rapid decrease in frequency of race T after susceptible Texas male sterile cytoplasm maize was replaced with normal cytoplasm maize following the 1970 southern leaf blight epidemic.