Pleistocene and Holocene fossils of the yellow-nosed cotton rat (Sigmodon ochrognathus) in New Mexico, Trans-Pecos Texas, and southern Chihuahua indicate past presence within the currently occupied geographic range, as well as occurrences at lower-elevation sites. Suggestions that post-Pleistocene colonization may account for occurrence of northern isolated populations cannot be falsified because extirpation followed by recolonization cannot be ruled out; however, current data indicate that such a scenario is unnecessary.