The cytochrome oxidase II (COII) genes in the mitochondrial DNA of 3 mosquito species (Anopheles sinensis, Aedes albopictus, and Culex quinquefasciatus) were amplified and sequenced. Both the gene order and direction of transcription were identical to those of other species of Anopheles, Aedes, and Culex. The polymerase chain reaction-amplified fragments in these mosquitoes were approximately 700 base pairs and the nucleotide sequences exhibited more than 82% similarity, whereas amino acids were more than 85% similar. The frequency of transitions was less than that of transversions. Four highly conserved segments of COII proteins are similar to those in other insects. These segments contain the major amino acid residues of cytochrome c oxidase involved in electron transport and ligand binding.