Pairs of strains of opposite mating type were isolated from a strain of Saccharomyces cerevisiae. From these isogenic strains, mitochondrially inherited resistant mutants to antimycin A and erythromycin were isolated. By using the two resistance genes as mitochondrial markers, it was proposed that the distribution of the mitochondrial genomes from zygotes to tetrads seemed not to be random but the genomes from either a or alpha parent would be selected with approximately equal frequencies after zygote formation and subsequently distributed uniparentally to meiotic products.