Chromosomes were analyzed from five subspecies of the ground squirrel, Spermophilus townsendi. The diploid chromosome number was 46 in S. t. vigilis and S. t. canus, 38 in S. t. idahoensis and S. t. mollis and 36 in S. t. townsendi. A chromosomally homogenous population of ground squirrels, separated from S. t. townsendi by the Yakima River and having 38 instead of 36 chromosomes, was recognized as a new subspecies, S. townsendi nancyae. Chromosomal evidence linked with zoogeography suggest that the subspecies of S. townsendi radiated from an ancestral population (2n = 46), probably originating in Oregon, with eventual spread north across the Columbia River (S. t. townsendi and nancyae) and southeast into Idaho, Utah and Nevada (S. t. mollis and idahoensis). Robertsonian centric fusion rather than dissociation of metacentric chromosomes receives strong zoogeographic support as the mechanism responsible for karyotype evolution in S. townsendi.
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