Cytoplasmically inherited chloramphenicol- and erythromycin-resistant mutants were obtained in three unrelated and two isogenic haploid strains of yeast. The bias favoring the transmission of these resistance alleles in crosses to the isogenic strains was compared on two levels: on the population level by means of observing random diploid progeny from mass matings, and on the zygote level by zygotic pedigree analyses. The genetic basis of this bias was determined by tetrad analysis. Our results suggest that 1. an intracellular selection mechanism operates within zygotes to determine the degree of bias; 2. the selection mechanism operates differently with respect to the two loci, C and E, under consideration; and 3. the selection mechanism is controlled by a set of nuclear genes. Other models which have been suggested to explain bias are critically examined in light of our results.