A number of different crosses between strains of Saccharomyces cerevisiae differing in mitochondrial genotype are analyzed with respect to the extent to which individual zygotes transmit mitochondrial genes from one parent or the other. Many crosses produce two or more distinct classes of zygotes in this respect. Some crosses produce a high frequency of uniparental zygotes, which transmit mitochondrial genes exclusively or nearly so from one parent. Such zygotes cannot be accounted for in terms of an unequal input of mitochondrial DNA molecules from the two parents; they indicate that mitochondrial DNA from one parent is selectively replicated, or mitochondrial DNA from the other parent is selectively destroyed, in the zygote. Multiple zygote classes, and uniparental zygotes, are seen in studies of mitochondrial and chloroplast inheritance in other organisms, and may have a common explanation.