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Mice of the Peromyscus aztecus species group occur at mid to high elevations in several mountain ranges in the highlands of Middle America (Mexico and Central America), a region of high endemicity. We examined the biogeography of this group by conducting phylogenetic analyses of 668 bp of the mitochondrial cytochrome b (cyt b) gene. Phylogenetic analyses under both parsimony and likelihood frameworks produced the same topologies, but estimates of nodal support were artificially high in weighted parsimony analyses. This difference is attributed to the inability of parsimony to optimize branch lengths when evaluating topologies. These data indicate that the P. aztecus-like populations from south and east of the Isthmus of Tehuantepec currently assigned to P. a. oaxacensis represent a distinct species, with genetic distances as high as 0.091. In addition, P. hylocetes is strongly divergent from Mexican …