The eastern fox squirrel (Sciurus niger) occurs naturally over most of eastern North America. The striking patterns of geographic variation in size and coat color displayed by this species are consistent with a hypothesis of southward range contraction and isolation in 2 refugia (in Texas and Florida) during the Last Glacial Maximum, followed by northward range expansion after the glaciers receded. Similar hypotheses have been proposed to explain the patterns in phylogeographic structure exhibited by many plants and animals in eastern North America. We analyzed DNA sequence variation in a 402–base-pair segment of the mitochondrial cytochrome-b gene in populations throughout the species' range. Despite our broad geographic sampling, we failed to detect any phylogeographic structure. Unique haplotypes differed from high-frequency haplotypes by only 1 or 2 base pairs, producing a starlike phylogeny of haplotypes. Genetic variation within populations and the species as a whole was characterized by high haplotype diversity and low nucleotide diversity. Taken together, our data indicate that the eastern fox squirrel underwent a rapid range expansion and rapid morphological divergence within the past 14,000 years.