I am grateful to Professor E. Raymond Hall for critical reading of the manuscript and helpful suggestions, to Dr. Rollin H. Baker and various of my fellow students at the Museum of Natural History for stimulating comments pertinent to the problems involved in this study, to my wife, Justine Anderson, for assistance in the preparation of the manuscript, to numerous members of field parties from the Museum of Natural History, who collected much of the material studied, and to the curators and other persons, at the museums listed below, who courteously made specimens available for study. The field work of the Museum of Natural History was assisted by the Kansas University Endowment Association. A National Science Foundation Fellowship made it possible for me to visit the museums listed below. An honorarium awarded by the American Society of Mammalogists enabled me to present this paper at the 34th Annual Meeting of the Society, in June of 1954. Unless otherwise indicated, specimens are in the University of Kansas Museum of Natural History. Specimens in other museums are designated beyond as follows: American Museum of Natural History (AMNH); Museum of Zoology, University of Michigan (Mich); Chicago Natural History Museum (Chi); United States National Museum (USNM); Biological Surveys Collection (USBS); Museum of Vertebrate Zoology, Berkeley, California (MVZ); Colorado Museum of Natural History, Denver (Denv); E. R. Warren collection, Colorado College, Colorado Springs (ERW); University of Colorado Museum, Boulder (UC). ACCOUNTS OF SUBSPECIES Microtus pennsylvanicus modestus (Baird) _Arvicola modesta_ Baird, Repts. Expl. and Surv...., pt. 1, Mammals, p. 535, July 14, 1858. _Microtus pennsylvanicus modestus_, Bailey, N. Amer. Fauna, 17:20, June 6, 1900. _Type._--Immature specimen (sex not specified), skin and skull, number of skin 594, number of skull 1717, deposited in the collections of the Smithsonian Institution, obtained by F. Kreutzfeldt, Sawatch Pass, Rocky Mountains [=Saguache Pass or Cochetopa Pass, Saguache County, Colorado], exact date unknown. I have not examined the type specimen. _Range._--Northern New Mexico, and southern Colorado (see list of specimens and Fig. 2).