Individuals heteroplasmic for mitochondrial DNA were observed in seven Mytilus populations at frequencies varying between 7 and 38%. The majority of heteroplasmic individuals possessed two genomes, referred to as the F and M genomes, which appear to exhibit base sequence divergence estimated to be about 10%. All homoplasmic individuals examined possessed the F genome. A large sex difference in the incidence of heteroplasmy was observed with 37% of males but only 5% of females possessing the M genome. Analysis of mtDNA extracted from different tissues provided evidence that the F genome predominates in female gonads and the M genome predominates in male gonads. Biparental transmission of mtDNA is considered as an explanation of these results but is not supported by a statistical analysis of the association of genotypes within heteroplasmic individuals. Because the F genome predominates in the gonads of most females, it is suggested that the M genome must have a replicative advantage to be detected in large quantity within individuals.