Competitions between matched pairs of diploid strains of Saccharomyces cerevisiae, one capable of undergoing sexual recombination (MAT-heterozygous) and the other not (MAT-homozygous), have proved useful for measuring the effects of mitotic and meiotic recombination and DNA repair on competitive ability in this organism. Overall competitive differences between the strains can be enhanced by converting them to petites (aerobic respiration incompetent). Here we report the results of competitions between pairs of strains that also differ in their ability to undergo mismatch repair. In petite strains, the growth rates of mismatch-repair defective strains declined over time regardless of their MAT genotype. Mismatch-repair proficient MAT-heterozygous strains did not show a decline, while repair-deficient MAT-homozygous strains did. The decline appears to be due to the accumulation of deleterious mutations of small effect, which can be corrected by MAT-heterozygous strains having intact mismatch repair. The relative competitive abilities of MAT-heterozygous and MAT-homozygous strains diverged during the course of the competitions, and the variance of this divergence increased significantly when mismatch repair was defective. This large stochastic component indicates that a relatively small number of deleterious mutations may be involved. The accumulation of deleterious mutations and their subsequent repair may have a bearing on the origin of sex in this organism.