Cytochrome c oxidase subunit II (COII), encoded by the mitochondrial genome, exhibits one of the most hetero- geneous rates of amino acid replacement among placental mammals. Moreover, it has been demonstrated that cytochrome c oxidase has undergone a structural change in higher primates which has altered its physical interaction with cytochrome c. We collected a large data set of CO11 sequences from several orders of mammals with emphasis on primates, rodents, and artiodactyls.