Nucleotide sequence data from the mitochondrial DNA (mtDNA) cytochrome-b gene and allozymic data were used to infer the evolutionary and biogeographic histories of New World tree squirrels of the genus Tamiasciurus. Phylogenetic analyses of the cytochrome-b data support the existence of 3 mtDNA lineages within Tamiasciurus: a western lineage consisting of populations of T. douglasii from western British Columbia (Canada), Washington, Oregon, and California, and T. mearnsi from northern Baja California (Mexico); a southwestern lineage consisting of populations of T. hudsonicus from New Mexico and Arizona; and a geographically widespread lineage comprising populations of T. hudsonicus from the remainder of the species' range. Levels of mtDNA sequence variation observed within and among populations of Tamiasciurus were small (0-2.4%), suggesting that contemporary geographic patterns of genetic variation in Tamiasciurus have been established relatively recently (i.e., in the Late Pleistocene). Allozyme analyses also support a close relationship among extant populations of Tamiasciurus. No fixed allelic differences were observed among the 3 recognized species and interspecific genetic distances (Nei's D) were substantially less than those typically observed between sibling species. Although differing from the current taxonomy in several respects, geographic patterns of genetic variation observed within Tamiasciurus are similar to those observed in a variety of North American boreal forest taxa and most likely reflect effects of forest fragmentation associated with glacial cycles of the Pleistocene.