Historically, specimens representing the Peromyscus boylii species group (from montane regions of western and southwestern Mexico) have been referred to as P. boylii or P. levipes. However, previous studies indicated that specimens from eastern Nayarit possessed a karyotype and mitochondrial DNA haplotype distinct from other members of the P. boylii species group. Together, these data precluded an assignment of these specimens to any currently recognized taxon in the P. boylii species group. Availability of additional specimens from this region provided an opportunity to reevaluate the taxonomic status of this complex. Analyses of 18 morphological characters demonstrated that specimens from eastern Nayarit possessed a significantly longer length of the nasals relative to the other species. Further, phylogenetic analyses (parsimony and likelihood) of DNA sequences obtained from the mitochondrial cytochrome-b gene indicate these specimens form a monophyletic clade embedded within a strongly supported clade containing representatives of P. beatae, P. levipes, and P. schmidlyi. Together, these results indicated that specimens from the Sierra Madre Occidental region of Nayarit represent an undescribed species of Peromyscus.