Peromyscus interparietalis is considered to be specifically distinct from P. guardia, of which it was originally considered a subspecies. Each species includes three subspecifically distinct populations, each endemic to an island in the northern Gulf of California, Baja California, Mexico. Two subspecies of P. interparietalis and one of P. guardia are newly described here. The mice are widespread on the islands, apparently favoring no particular microhabitat. Reproduction takes place in early spring.