Mammalian mitochondrial DNA (mtDNA) is a highly polymorphic, high-copy-number genome that is maternally inherited. New mutations in mtDNA segregate rapidly in the female germline due to a genetic bottleneck in early oogenesis^1–3 and as a result most individuals are homoplasmic for a single species of mtDNA. Sequence variants thus accumulate along maternal lineages without genetic recombination. Most of the extant variation in mtDNA in mammalian populations has been assumed to be neutral with respect to selection⁴; however, comparisons of the ratio of replacement to silent nucleotide substitutions between species suggest that the evolution of mammalian mtDNA is not strictly neutral⁵. To test directly whether polymorphic mtDNAs behave as neutral variants, we examined the segregation of two different mtDNA genotypes in the tissues of heteroplasmic mice. We find evidence for random genetic drift in some tissues, but in others strong, tissue-specific and age-related, directional selection for different mtDNA genotypes in the same animal. These surprising data suggest that the coding sequence of mtDNA may represent a compromise between the competing demands of different tissues and point to the existence of unknown, tissue-specific nuclear genes important in the interaction between the nuclear and mitochondrial genomes.