The Texas mouse, Peromyscus attwateri, is a relatively habitat-specific species that exhibits a discontinuous distribution across the south-central United States. To examine the evolutionary history and contemporary population genetic structure of P. attwateri we sequenced an 1,100-base pair fragment of the mitochondrial cytochrome b gene for 210 individuals from 22 localities, resulting in 89 unique haplotypes. Low nucleotide diversity (π  =  0.0043) and a median-joining haplotype network indicated low levels of divergence among haplotypes with little geographic structure. Demographic analyses indicated the presence of 2 significant range expansions: the 1st coinciding with the end of the last glacial maximum of the Pleistocene approximately 14 thousand years ago (kya) and the 2nd more recent expansion during the hypsithermal (9–5 kya) of the Holocene. Coalescent simulations under a model of no gene flow indicated that the lack of divergence among populations most likely is attributable to incomplete lineage sorting, and the observed gene flow statistic (s  =  116) suggests that populations became isolated approximately 7.5 kya. This finding substantiates the hypothesis that the thermal maximum of the hypsithermal may have had a significant impact on small mammals as well as other organisms in the south-central United States.