Yeast cells contain many copies of mitochondrial (mit) genomes. The question we tried to answer was how mit− mutations occurring in one genome as a result of mutagenic treatment might yield homoplasmic mutant cells. Three processes were considered. First, that these cells originate by segregation of mutant and standard alleles during cell division. Secondly, that they originate through intracellular selection, for which cell division is not required. Thirdly, that recombination involving the mutant and standard alleles is non-reciprocal and unidirectional mit+ → mit− so that the mutant allele is spread into the entire population of mitochondrial genomes within a cell, thus making it homoplasmic mit−. The results indicate that the first process, although efficiently producing homoplasmic cells from heteroplasmic zygotes (for review see Birky, 1978), seems to play only a minor, if any, role in producing homoplasmic mutant progenies from mutagenized cells. The most important is the second process, that is, intracellular selection occurring in cells which have one or a few genomes carrying mit− mutations, while the remaining genomes are irreversibly damaged. The third process, unidirectional mit+ → mit− conversion, does not seem to play any part.