Specimens of _M. montanus_ from northern New Mexico have been referred previously to _M. m. arizonensis_. When he named _M. m. fusus_, Hall mentioned its resemblance to _arizonensis_ in reddish coloration, but pointed out that _fusus_ is less reddish. Of six specimens from Valle Santa Rosa, Jemez Mountains (USBS), 8500 ft., Rio Arriba County, New Mexico, three are immature, and the skulls of the remaining specimens are damaged. In reddish color and relatively large size these few specimens resemble _arizonensis_ more than _fusus_ although the locality of occurrence is closer to the geographic range of the northern _fusus_ than to that of _arizonensis_. The identification of these specimens as _arizonensis_ is provisional; additional specimens are needed from the area, 200 miles wide, which separates the ranges as now known of _arizonensis_ in Arizona from the occurrence in New Mexico. There is a single specimen from this area, the damaged skull of which prevents conclusive identification. The specimen is either _M. montanus_ or _M. mexicanus_, and is from Nutria, on the southern edge of the Zuni Mountains (USBS). Detailed comparison of _fusus_ and _arizonensis_ is not attempted here although it may be stated that in several characters _fusus_ is intermediate between _arizonensis_ to the south and _nanus_ to the north. _Specimens examined._--Total, 89, distributed as follows (abbreviations for collections are given in the account of _M. m. nanus_; localities that are not represented in Fig. 1 because overlapping or crowding of the symbols would result are Italicized): COLORADO: _Pitkin Co._: 5 mi. W Independence Pass, 11,000 ft., 1 (Chi). _Lake Co._: _Independence Pass, 12,095 ft._, 2 (Chi). _Gunnison Co._: _Gothic_, 2 (USBS); Decker's Ranch, Crested Butte, 2 (AMNH); Almont, 3 (USBS). _Montrose Co._: Coventry, 5 (USBS 4, AMNH 1). _Saguache Co._: Cochetopa Pass and environs, 44 (USBS 22). _Hinsdale Co._: Ruby Lake, 1 (USBS). _Mineral Co._: 3 mi. E Creede, 1; 23 mi. S, 11 mi. E Creede, 9300 ft., 7. _La Plata Co._: Florida, 6800 ft., 1. _Conejos Co._: 1 mi. S, 19 mi. W Antonito, 10,200 ft., 3; _4 mi. S, 23 mi. W Antonito_, 1; _5 mi. S, 24 mi. W Antonito, 9600 ft._, 9. NEW MEXICO: _Rio Arriba Co._: 6 mi. W Hopewell, 9900 ft., 6 (USBS); _Tusas River, 8700 ft._, 1 (USBS). TABLE 1. AVERAGE MEASUREMENTS, IN MILLIMETERS, OF ADULTS OF MICROTUS MONTANUS. Key to column headings: A: No. of individuals averaged B: Total length C: Length of tail D: Length of hind foot E: Condylobasilar length F: Alveolar length of upper molar tooth-row G: Zygomatic breadth H: Lambdoidal breadth I: Prelambdoidal breadth