Late Pleistocene and Recent records of Neofiber alleni True, the round-tailed muskrat, are confined to Florida and Georgia. Extinct middle Pleistocene species are known from the Great Plains and middle Atlantic states. The middle Pleistocene species N. leonardi Hibbard, previously known only from Texas and Kansas, is here recognized in Florida and West Virginia. The genus became restricted to the Southeast as a result of the climatic changes during the last two glacial intervals. Common morphological features of the first lower molar indicate that N. leonardi is the ancestor of N. alleni.