Inheritance of mitochondrial DNA in animals was thought to be strictly maternal. Recently, evidence for incidental paternal mtDNA leakage was obtained in hybrid crosses of Drosophila and mice. In mice, the frequency of paternal mtDNA contributions was estimated at 10(-4), compared with maternal contributions. The common occurrence in the marine mussel Mytilus of heteroplasmic individuals with two or more types of highly diverged mtDNA molecules was interpreted as strong evidence for biparental mtDNA inheritance by some, but not by others. We report here results from pair-matings involving two species of mussels, Mytilus edulis and Mytilus trossulus. Extensive contribution of paternal mtDNA, amounting to several orders of magnitude higher than that inferred for Drosophila or mice, was observed in both intra- and interspecific crosses.