_Microtus nanus_, Bailey, N. Amer. Fauna, 17:30, June 6, 1900 (part). _Microtus montanus caryi_ Bailey, Proc. Biol. Soc. Washington, 30:29, February 21, 1917. _Microtus nanus nanus_, Cary, N. Amer. Fauna, 42:43, October 3, 1917 (part). _Type._--Adult male No. 23853/31253, U. S. National Museum, Department of Agriculture collection, from Pahsimeroi Mountains, Custer County, Idaho; obtained by C. Hart Merriam and Vernon Bailey, September 16, 1890. _Range._--Idaho; southwestern Montana; most of the southwestern half of Wyoming; southward to central Colorado. See figure 1. _Comparisons._--Comparisons with subspecies named as new in this paper will be found in the accounts of those subspecies beyond. From _Microtus montanus fusus_ Hall, the subspecies to the south, _M. m. nanus_ from Idaho differs as follows: averages smaller; slightly darker and less reddish and less yellowish in color; slightly wider braincase (see measurement of prelambdoidal breadth); larger bullae. _Measurements._--Average (= arithmetical mean) measurements of 34 specimens, both male and female, from several localities in eastern Idaho are: total length, 151; length of tail, 39; hind foot, 19.2; condylobasilar length of the skull, 25.0; zygomatic breadth, 15.0; alveolar length of upper molar tooth-row, 6.4; prelambdoidal breadth, 8.9; and lambdoidal breadth, 11.7. Average and extreme measurements of six adult males from near Pocatello, Bannock County, Idaho, and nine adult males from near Afton, Lincoln County, Wyoming, are, respectively, as follows: total length, 143(135-150), 163(143-179); length of tail, 35.1(33-38), 42.8(36-49); caudal index, 32.0(28.0-33.1), 35.7(30.6-41.9); hind foot, 18.9(18-20), 18.8(17-20); condylobasilar length of skull, 24.4(24.0-26.0), 25.6(24.5-26.2); alveolobasilar length, 14.1(13.7-14.5), 14.6(13.8-15.0); palatilar length, 13.2(12.9-13.6), 13.8(13.2-14.5); alveolar length of upper molar tooth-row, 6.3(6.1-6.5), 6.3(6.0-6.6); depth of braincase, 7.7(7.5-7.9), 8.0(7.7-8.3); lambdoidal breadth, 11.4(11.0-11.7), 12.0(11.3-12.7); prelambdoidal breadth, 9.1(8.6-9.4), 8.7(8.0-9.4); zygomatic breadth, 14.3(13.8-14.7), 15.3(14.4-16.3); interorbital breadth, 3.6(3.5-3.7), 3.5(3.3-3.7). The average length of the nasal bones in the series from Pocatello is 7.1 mm. The averages, which have not been included in Table 1, for three measurements of the series from Carbon County, Wyoming, are as follows (Encampment, males; Encampment, females; Savery, males; and Savery, females, respectively): alveolobasilar length, 14.4, 14.3, 14.5, 14.3; interorbital breadth, 3.5, 3.4, 3.5, 3.4; depth of braincase, 7.8, 7.6, 7.9, 7.6. Additional measurements are included in Table 1 for other series.