To evaluate phylogenetic relationships within the Peromyscus mexicanus species group, we examined allozymic variation at 28 presumptive genetic loci for nine members of the group (P. guatemalensis, P. gymnotis, P. megalops, P. melanocarpus, P. mexicanus, P. nudipes, P. ochraventer, P. yucatanicus, and P. zarhynchus), representatives of four other species groups within the subgenus Peromyscus (boylii, leucopus, maniculatus, and truei species groups), and two outgroups to the subgenus Peromyscus (Podomys floridanus and Megadontomys thomasi). Phenetic analysis of genetic distances generally was concordant with cladistic analyses of discrete character states. Among the nine species usually included within the mexicanus group, six (P. guatemalensis, P. gymnotis, P. mexicanus, P. nudipes, P. yucatanicus, and P. zarhynchus) were genically similar, and relationships among these taxa generally were undefined in the cladistic analysis. Within this group, a population of P. mexicanus shared two derived allelic character states with P. gymnotis and as currently defined, P. mexicanus appears paraphyletic. P. megalops, P. melanocarpus, and P. ochraventer were neither associated closely with other species in the subgenus Peromyscus nor with each other, phenetically or cladistically. Based on these data, the mexicanus species group, as currently defined, is composite. Our data agree with the removal of P. megalops, P. melanocarpus, and P. ochraventer from the mexicanus group, but not with the assignment of P. megalops and P. melanocarpusto a single species group.