The sequence of the mitochondrial cytochrome oxidase I (COI) gene was analyzed in 168 individuals from seven Aphis (Hemiptera: Aphidini: Aphidina) species: Aphis gossypii Glover, Aphis glycines Matsumura, Aphis spiraecola Patch, Aphis craccivora Koch, Aphis fabae Scopoli, Aphis rumicis (L.), and Aphis nerii Boyer de Fenscolombe. One other Aphidina species, Toxoptera odinae (van der Goot), and two Macrosiphini species, Acyrthosiphon pisum (Harris) and Myzus persicae (Sulzer), were analyzed as out-groups. The coding region of the gene contains 1,563 bp in all of the species, and 272 sites were variable among the Aphis species. DNA sequence identities among individual aphids ranged from 91.4 to 100%, with greater identities among aphids in the same species (96.5–100%) than between different species (91.4–95.3%). Compared with some other species, A. gossypii showed little divergence in either nucleotide sequences or haplotypes, although intraspecific cytochrome oxidase I (COI) sequence divergence was observed in A. gossypii clones isolated from Commelina communis L. and Rubia cordfolia L. In the phylogenetic tree, every species formed a robust clade. The pairs A. gossypii and A. glycines, and A. rumicis and A. fabae showed robust sister relationships. T. odinae was located within the Aphis species clade, and this suggested that the Toxoptera classification should be reconsidered in the light of molecular data.