Mitochondria isolated from rat liver cells or mycoplasma-free HeLa cells contain a single DNA polymerase activity which is closely related to, or identical to, the DNA polymerase gamma activity found in the homologous cell. In rat liver cells, about 16% of the total cytoplasmic gamma-polymerase activity is found associated with mitochondria and in HeLa cells about 20% of the total cellular gamma-polymerase is mitochondria associated. Since mitochondria possess no unique DNA polymerase activity, the number of DNA polymerases now known in mammalian cells is reduced, from the previously proposed four enzymes, to three--DNA polymerases alpha, beta, and gamma.