_Comparisons._--From _M. p. uligocola_, the subspecies to the southeast, _M. p. pullatus_ differs as follows: relatively darker in southern part of its range (see Figure 1); smaller, tail relatively longer. In 6 of 7 pairs of skulls compared of _pullatus_ (from Lincoln Co., Wyoming) and _uligocola_ (from Larimer Co., Colorado), _pullatus_ had relatively broader nasals (Confidence Limit .85); _uligocola_ had larger maxillary septa (C. L. .97) and larger molar teeth (C. L. .90). From _insperatus_, the subspecies to the east, _pullatus_ differs as follows: both summer and winter pelage darker; tail longer both actually and relatively; upper molar tooth-row shorter. Ten pairs of skulls of specimens from near Afton, Wyoming, representing _pullatus_, and from northeastern Wyoming, representing _insperatus_, revealed no significant differences in the features observed by the "method of pairs". Although not compared in detail with the subspecies to the north, _M. p. drummondi_ (Audubon and Bachman), examination of specimens from western Montana and the accounts of other authors indicate that topotypes of _pullatus_ are darker, longer-tailed, slightly larger-skulled and perhaps longer over all. _Remarks._--In this subspecies there is a cline in color from dark in extreme southwestern Wyoming to pale in north-central Wyoming and Montana as the range of _M. p. insperatus_ is approached. There is thus a broad zone of intergradation in color and the line separating the subspecies must be drawn somewhat arbitrarily. In Wyoming the most distinct break in this cline is in the Big Horn Basin and if a detailed study of the species were made in Montana the break would probably be found where the mountains meet the plains, roughly as shown in Figure 1. There is a similar cline in western Montana in color. The mice are paler farther north as one approaches the Canadian border although they do not become so pale as _insperatus_. Darkness is a characteristic of several non-adjacent subspecies of _Microtus pennsylvanicus_, for example _M. p._ _kincaidi_ Dalquest in central Washington (Dalquest, 1948:347), _M. p. finitus_, and _M. p. nigrans_ Rhoads in eastern Virginia, but these subspecies presumably differ in other characters.