We determined nucleotide sequences of homologous 0.9-kb fragments of mitochondrial DNAs (mtDNAs) derived from four species of old-world monkeys, one species of new-world monkeys, and two species of prosimians. With these nucleotide sequences and homologous sequences for five species of hominoids, we constructed a phylogenetic tree for the four groups of primates. The phylogeny obtained is generally consistent with evolutionary trees constructed in previous studies. Our results also suggest that the rate of nucleotide substitution for mtDNAs in hominines (human, chimpanzee, and gorilla) may have slowed down compared with that for old-world monkeys. This evolutionary feature of mitochondrial genes is similar to one found in nuclear genes.