The giant ground squirrel Paenemarmota barbouri is known from localities of Blancan age in Nebraska, Kansas, Texas, Arizona, and Chihuahua. It has been found in beds of late Hemphillian age in Chihuahua and of questionable Hemphillian age in Michoacan. Comparable in size to a large living beaver, Paenemarmota is the largest known ground squirrel and is nearly twice as large as the largest living ground squirrels. Although some of its features are primitive, in most respects it is a "modern" ground squirrel. Analysis of the morphology of Paenemarmota shows greatest similarity to living spermophiles, particularly to the subgenus Spermophilus. It could be considered a primitive member of this group of ground squirrels except for the large size of the upper and lower fourth premolars, a generic character of modern marmots. The large fourth premolars are exceptions to the general evolutionary trend in the ground squirrels, and only in Marmota and, particularly, Paenemarmota are these teeth larger than the first molars. Whether this represents parallel development or phylogenetic relationship cannot be decided without additional fossil evidence.