_Diagnosis._--Size small for the species (see measurements). Color: Essentially as in _Microtus montanus nanus_. Skull: Small, slender, and comparatively smooth; rostrum moderately depressed distally; nasals moderately inflated distally and extending posteriorly not quite to tips of premaxillary tongues; nasals usually truncate posteriorly, but rounded in some individuals; premaxillary tongues terminating posteriorly in a short medial spine; zygomatic arches lightly constructed and usually more widely spreading posteriorly than anteriorly; interparietal comparatively long and terminating in a small, but distinct, medial spine, otherwise approximately rectangular in shape; exposed parts of upper incisors short and, for the species, only slightly procumbent; molar dentition weak and, in most specimens, especially so posteriorly; tympanic bullae large and well inflated, especially ventrolaterally; basioccipital narrow owing to the encroachment of the tympanic bullae. _Comparison._--Among named forms, _Microtus montanus pratincolus_ most closely resembles _Microtus montanus nanus_. The geographic range of _M. m. nanus_ adjoins that of _M. m. pratincolus_ on three sides; there is no conspecific subspecies adjoining the range of _M. m. pratincolus_ on the north. From _M. m. nanus_, _M. m. pratincolus_ differs as follows (measurements are all of males, those of _M. m. nanus_ being of nine topotypes and near topotypes from central Idaho): size smaller (149 mm. as opposed to 165), tail shorter (37 as opposed to 39), hind foot shorter (19 as opposed to 20), upper molar series shorter (expressed as a percentage of basilar length, 25.5 as opposed to 26.3), mastoidal region broader (expressed as a percentage of basilar length, 48.6 as opposed to 46.7), braincase slightly more vaulted (depth of braincase expressed as a percentage of basilar length, 31.3 as opposed to 30.4) and more inflated laterally; tympanic bullae more inflated, this inflation being the most conspicuous difference between the two subspecies. The tympanic bullae of _M. m. pratincolus_ have approximately a fourth more volume than those of _M. m. nanus_. _Remarks._--Northwardly in the Bitterroot Valley, specimens of _M. m. pratincolus_ morphologically approach _M. m. nanus_, especially in the reduced degree of inflation of the tympanic portion of the bullae. On geographic grounds we think that the geographic range of _M. m. pratincolus_ extends southward to the southern end of the Bitterroot Valley; we have not seen specimens from that area. Although we have not examined the specimen reported upon by Davis (Murrelet, 18:26, September 4, 1937) from Canyon Creek, "a few miles west of Hamilton", Montana, we think that it will be found to belong to _M. m. pratincolus_.