Peromyscus slevini Mailliard (1924) is endemic to Santa Catalina Island, a small oceanic island in the southern Sea of Cortez. The species has been variously associated, formally or informally, with 2 subgenera—Haplomylomys and Peromyscus—and 5 species-groups—californicus, eremicus, maniculatus, and boylii or mexicanus. Based upon our taxonomic evaluations, we document that 2 species of Peromyscus, P. slevini and P. fraterculus, occur among the samples that authors have identified as P. slevini, clarify the diagnosis of P. slevini proper and allocate it to membership in the P. melanophrys species-group, and discuss biogeographic implications of this proposed relationship.