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Molecular data have been used to show northward post-Pleistocene range expansions from a refugium in the southeastern United States for several mammal species. Fossil and historical records indicate that gray foxes (Urocyon cinereoargenteus) were not present in the northeastern United States until well after the Pleistocene (ca. 900). To test the hypothesis that gray foxes experienced a post-Pleistocene range expansion we conducted a phylogeographic analysis of gray foxes from across the eastern United States. We sequenced a variable portion of the mitochondrial control region (411 base pairs) from 229 gray fox tissue samples from 15 states, representing the range of all 3 East Coast subspecies. Phylogeographic analyses indicated no clear pattern of genetic structuring of gray fox haplotypes across most of the eastern United States. However, when haplotype frequencies were subdivided into a northeastern and a southern region, we detected a strong signal of differentiation between the Northeast and the rest of the eastern United States. Indicators of molecular diversity and tests for demographic expansion confirmed this division and suggested a very recent expansion of gray foxes into the northeastern states. Our results support the hypothesis that gray foxes 1st colonized the Northeast during a historical period of hemisphere-wide warming, which coincided with the range expansion of deciduous forest. We present the 1st study that analyzes the phylogeographic patterns of the gray fox in the eastern United States. [ABSTRACT FROM AUTHOR]