The catalytic subunit of the holoenzyme appears to be of significant size, approximately 140 kD in Xenopus laevis a similar situation may prevail for D. melanogaster in both these cases there appears to be a stoichiometric, smaller subunit present in the most highly purified preparations. Human mtDNA polymerase has been partially purified and has recently been cloned by virtue of its conservation to yeast mtDNA polymerases. This significant advance should permit a comprehensive study of the structure, functional parameters, and intermitochondrial distribution of this central activity in mtDNA replication