Specimens examined of _M. m. nanus_ from eastern Idaho and Montana are as follows: IDAHO: _Custer Co._: Challis, 7 (USBS); Mill Creek, Challis Nat. Forest, 1 (USBS); Pahsimeroi Mts., 12 (USBS); Lost River Mts., 1 (USBS). _Fremont Co._: N fork Snake River, 10 mi. SW Island Park, 6200 ft., 2 (AMNH); Black Springs Creek, 4 mi. W Ashton, 5200 ft., 1 (AMNH); 5 mi. W St. Anthony, 5000 ft., 1 (AMNH). _Camas Co._: Camas Prairie, Corral, 5100 ft., 2 (USBS). _Blaine Co._: Alturas Lake, 3 (USBS); _Sawtooth Lake_, 2 (USBS); Craters of the Moon, Laidlow Park, 2 (Mich); Ticura, 10 mi. S Picabo, 1 (USBS); 19 mi. NE Carey (Lava Lake), 8 (Mich). _Butte Co._: _26 mi. SW Arco_, 12 (Mich). _Bingham Co._: Shelley, 6 (USBS). _Bonneville Co._: 10 mi. SE Irwin, 4 (USBS). _Owyhee Co._: Three Creeks, 3 (USBS). _Twin Falls Co._: Castleford Fenced Plot, 11 mi. W, 9 mi. S Twin Falls, 1. _Minidoka Co._: _Heyburn_, 2 (USBS). _Cassia Co._: 2 mi. S, 2 mi. W Burley, 5. _Bannock Co._: Pocatello, 23 (USBS 4); Swan Lake, 1 (USBS). _Bear Lake Co._: Montpelier Creek, 6700 ft., 3 (USBS). MONTANA: _Gallatin Co._: W. Fork of W. Fork, Gallatin River, 1 (USBS). _Park Co._: Lamar River, 7000 ft., 1 (USBS); Gardiner, 1 (USBS). _Sweet Grass Co._: 14 mi. S Big Timber, 1 (USBS); _McLeod_, 1 (USBS); West Boulder Creek, 18 mi. SE Livingston, 2 (USBS). =Microtus montanus codiensis=, new subspecies _Type._--Female, adult, skin and skull; No. 27578, Museum of Natural History, University of Kansas, from 3⅕ mi. E and ⅗ mi. S Cody, 5020 ft., Park Co., Wyoming; obtained on August 11, 1948, by James W. Bee, original number 18-8-11-48. _Range._--In northwestern Wyoming eastward from the Absaroka and Wind River ranges into the western part of the Big Horn Basin. _Diagnosis._--A relatively large _Microtus montanus_; tail actually and relatively long; hind foot actually but not relatively large; skull large; zygomatic expanse actually and relatively large; alveolobasilar length relatively large; upper molar tooth-row relatively long; color relatively light, not reddish. [Illustration: FIG. 1. Geographic range of _Microtus montanus_ in Wyoming, Colorado, and adjacent areas. The solid circles represent localities from which specimens have been examined; the hollow circles represent type localities. The ranges of subspecies in Utah are after Durrant, 1952. Guide to subspecies 1. _M. m. nanus_ 2. _M. m. codiensis_ 3. _M. m. zygomaticus_ 4. _M. m. fusus_ 5. _M. m. micropus_ 6. _M. m. nexus_ 7. _M. m. amosus_ 8. _M. m. rivularis_ ]