Specimens of the Peromyscus boylii species group occurring in the montane regions of Michoacán, México, historically have been assigned to P. levipes. However, previous studies have shown that some specimens from eastern Michoacán possessed mitochondrial DNA haplotypes and karyotypes that were distinct from P. levipes and other members of the P. boylii species group. Phylogenetic analyses (parsimony and likelihood) of additional DNA sequences obtained from the mitochondrial cytochrome-b gene indicated that specimens from central and eastern Michoacán and western Morelos formed a monophyletic clade that was sister to a clade containing representatives of P. beatae. Estimations of genetic divergence for members of these 2 sister clades exceeded 5% and were greater than most pairwise comparisons reported for other members of the P. boylii species group. Collectively, there are no discernable morphological differences between those specimens and other cryptic species in the P. boylii species group. Together, these results indicated that specimens from the Sierra Madre del Sur region of Michoacán, Morelos, and likely throughout the Neovolcanic Axis of the Estado de México represent an undescribed species of Peromyscus for which we propose the name Peromyscus kilpatricki.