This study is concerned primarily with mice from Wyoming and Colorado; I realize, however, that the physiographic and ecological conditions important to the distribution and subspeciation of _Microtus pennsylvanicus_ do not correspond to political boundaries. Geographic variation within these two states can be seen in proper perspective only when related to the neighboring areas and to previous studies. I have attempted to do this in the accounts of the subspecies. Approximately five months in the field in Wyoming and Colorado in the summers of 1950, 1951, 1952, and 1953 gave me a familiarity with the region that has helped to clarify the pattern of distribution. My study was based, in addition, on 762 specimens that are listed under "specimens examined" in the accounts of subspecies, and on comparative material from other states. Most of these specimens are skins with skulls but some are skins only and others are skulls only. Some localities are represented by too few adult individuals to permit significant comparisons. Owing to damaged skulls, certain measurements of some specimens were omitted from the calculations. If it seemed that the damaged skull was exceptionally large or small or a deviant in any other regard it was not used, in order not to bias the computed averages, which might be used in comparing proportions of the skulls. In the lists of specimens examined, localities that are omitted from Figure 2 because overlapping or undue crowding of the symbols would have resulted are _italicized_. [Illustration: FIGURE 2. Distribution of the subspecies of _Microtus pennsylvanicus_ in Wyoming and Colorado. Solid dots represent localities from which specimens have been examined, and triangles represent localities reported in the literature from which I have not examined specimens. The question mark in southern Colorado denotes a questionable record discussed in the text. A. _M. p. pullatus_ B. _M. p. insperatus_ C. _M. p. uligocola_ D. _M. p. finitus_ E. _M. p. modestus_ F. _M. p. aztecus_]