The study here reported upon is based on 1,187 specimens of _Microtus montanus_ from Wyoming, Colorado, Idaho and Montana, and on work in the field. I spent approximately four months in the field in this area, in the summers of 1950, 1951, and 1952. The specimens studied were arranged according to localities and the larger series were compared statistically. Each of two series, totaling 136 specimens, was studied intensively to ascertain the kind and range of variation within single populations. Twenty-seven measurements, various proportions based on these measurements, and differences in color were analyzed. Fifteen characters, judged to be most significant, were selected for use in comparing all series. In addition, certain characters that can not be expressed easily by measurements, such as inflation of the auditory bullae and the curvature of the zygomatic arch, were observed. The studies by A. B. Howell (1924) of variation in _Microtus montanus yosemite_ Grinnell in California and those by O. B. Goin (1943) of _Microtus pennsylvanicus pennsylvanicus_ (Ord) were useful. The external measurements are from the collectors' field labels. The measurements of the skull all were taken with dial calipers reading to a tenth of a millimeter. The anteroposterior measurements of the skull all were taken along the shortest line between the points specified below and are not necessarily along a line parallel to the long axis of the skull. These measurements were taken on the left side of the skull whenever possible. Some of the skulls are damaged and therefore some measurements could not be taken and are omitted in the computations. Measurements are in millimeters. The results of these studies were submitted to the Department of Zoology and the Graduate School of the University of Kansas in partial fulfillment of the requirements for the degree of Master of Arts (1952) and are available in manuscript form at the Museum of Natural History and the library of the University of Kansas. EXPLANATION OF MEASUREMENTS Caudal index.--the length of the tail expressed as a percentage of the length of the head and body. The length of the head and body is the collector's measurement of total length less the length of the tail. CRANIAL MEASUREMENTS OF LENGTH. Condylobasilar length.--from the exoccipital condyle to the most posterior point on the border of the alveolus of the upper incisor. Alveolobasilar length.--from the posterior border of the alveolus of the third upper molar to the posterior border of the alveolus of the incisor. Palatilar length.--from the anteriormost part of the posterior border of the bony shelf of the palate to the posteriormost part of the alveolus of the incisor. Alveolar length of upper molar tooth-row.--from the most posterior point of the alveolus of the third upper molar to the most anterior point of the alveolus of the first upper molar.