A new, relatively large species of Niptomomys is described from the late Wasatchian of the Bighorn Basin of Wyoming. The importance of a stratigraphic approach to problems of species-level phylogeny is stressed, and then applied to an investigation of the evolutionary history of Niptomomys. The new Niptomomys species may have evolved gradually from early Wasatchian Niptomomys doreenae in the Bighorn Basin and vicinity, or it may have evolved elsewhere and replaced the earlier form relatively rapidly. The available evidence is not yet sufficient to distinguish between these two alternatives.