Numerous members of summer field parties from the Museum of Natural History at the University of Kansas collected most of the specimens studied and wrote field notes that have been helpful. I am grateful to these persons and to Professor E. R. Hall and Dr. R. H. Baker for their assistance and helpful suggestions. Specimens in the following museums were made available by their respective curators: Chicago Natural History Museum by Mr. Colin C. Sandborn, The Museum of Zoology at the University of Michigan by Dr. E. T. Hooper, The American Museum of Natural History by Mr. G. G. Goodwin, The United States National Museum by Dr. David H. Johnson and the Biological Surveys Collection by Miss Viola S. Schantz. A fellowship from the National Science Foundation made possible the studies at the museums other than at the University of Kansas.
_Arvicola (Mynomes) nanus_ Merriam, N. Amer. Fauna, 5:62, July 30, 1891.
_Microtus montanus nanus_, Hall, Proc. Biol. Soc. Washington, 51:131, August 23, 1938.
_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.