DNA sequence and morphometric data were used to re-evaluate the taxonomy and systematics ofPeromyscus pectoralis. Phylogenetic analyses (maximum likelihood and Bayesian inference) of DNA sequences from the mitochondrial cytochrome-b gene in 44 samples ofP. pectoralis indicated 2 well-supported monophyletic clades. The 1st clade contained specimens from Texas historically assigned toP. p. laceianus; the 2nd was comprised of specimens previously referable toP. p. collinus,P. p. laceianus, andP. p. pectoralis obtained from northern and eastern Mexico. Levels of genetic variation (~7%) between these 2 clades indicated that the genetic divergence typically exceeded that reported for other species ofPeromyscus. Samples ofP. p. laceianus north and south of the Río Grande were not monophyletic. In addition, samples representingP. p. collinus andP. p. pectoralis formed 2 clades that differed genetically by 7.14%. Multivariate analyses of external and cranial measurements from 63 populations ofP. pectoralis revealed 4 morpho-groups consistent with clades in the DNA sequence analysis: 1 from Texas and New Mexico assignable toP. p. laceianus; a 2nd from western and southern Mexico assignable toP. p. pectoralis; a 3rd from northern and central Mexico previously assigned toP. p. pectoralis but herein shown to represent an undescribed taxon; and a 4th from southeastern Mexico assignable toP. p. collinus. Based on the concordance of these results, populations from the United States are referred to asP. laceianus, whereas populations from Mexico are referred to asP. pectoralis (including some samples historically assigned toP. p. collinus,P. p. laceianus, andP. p. pectoralis). A new subspecies is described to represent populations south of the Río Grande in northern and central Mexico. Additional research is needed to discern ifP. p. collinus warrants species recognition.