DNA sequences from the mitochondrial cytochrome-b gene were used to examine the composition and phylogenetic relationships of the Peromyscus truei species group. Thirty-one individuals from the southwestern United States and Mexico were examined. Results indicated that 6 Peromyscus species (attwateri, difficilis, gratus, nasutus, pectoralis, and truei) compose the P. truei species group and that the group should be divided into 2 species assemblages. The relationship of the P. truei species group to the P. boylii and P. aztecus species groups was unresolved, indicating a close association of these 3 groups. In addition, levels of sequence divergence between sister taxa were higher than those reported for other sister species of Peromyscus.