Measurements of the type and one "more fully adult topotype" (as given by Bailey, _op. cit._) are: Total length, 125, 130; tail length, 31, 32; hind foot, 19, 19; basal length, 21.2, 21.8; length of nasals, 6.6, 7.0; zygomatic breadth, 12.5, 12.8; mastoid breadth, 11.3, 11.0; alveolar length of upper molar series, 5.4, 5.3. In every measurement the figures for Bailey's specimens are smaller than the average of the same measurement in the 20 adults from Pennington County, and, in most measurements, are even lower than the minimum of the latter series. Therefore, we conclude that the material available to Merriam (_op. cit._) and Bailey (_op. cit._) consisted of only subadults. In comparison with a series of 23 adult _Clethrionomys gapperi galei_ from 28 mi. E Lovell, Big Horn County, Wyoming, _C. g. brevicaudus_ has a slightly shorter tail, longer hind foot, greater basal and condylobasilar lengths, greater zygomatic and lambdoidal breadths and conspicuously longer nasals. In comparison with three adult _C. g. loringi_ from Elk River, Sherburne County, Minnesota, _C. g. brevicaudus_ has a greater total length, longer hind foot, greater basal length, conspicuously greater zygomatic and lambdoidal breadths, much longer nasals, and a narrower rostrum. _Clethrionomys gapperi brevicaudus_, although isolated geographically and although morphologically more distinct than many of the currently recognized subspecies of _C. gapperi_, is probably best arranged as a subspecies of _C. gapperi_ rather than as a full species. In certain characters, such as interorbital breadth and breadth of rostrum, it is intermediate between _C. g. galei_ and _C. g. loringi_, but it resembles _C. g. galei_ more than it does any other named kind. _Specimens examined._--Total, 66. Unless otherwise indicated, specimens from Wyoming are in the University of Kansas Museum of Natural History and specimens from South Dakota are in the University of Michigan Museum of Zoology. Specimens are distributed as follows: +Wyoming+: _Crook County_: 3 mi. NW Sundance, 5900 ft., 3. _Weston County_: 1-1/2 mi. E Buckhorn, 6150 ft., 21; 12 mi. SE Newcastle, 1 (Univ. Michigan). +South Dakota+: _Pennington County_: 1/2 mi. E Rochford, 1; 17 mi. NW Custer, 1; 16 mi. NW Custer, 20; 16 mi. SW Rapid City, 1; 3 mi. SE Hill City, 2; 4 mi. SE Hill City, 13; 5 mi. SE Hill City, Harney Peak, 7240 ft., 1. _Custer County_: 1-1/2 mi. E Sylvan Lake, 1. +Clethrionomys gapperi uintaensis+ Doutt 1897. _Evotomys gapperi galei_, Bailey, Proc. Biol. Soc. Washington, 11:127 (part from Uinta Mts. of Wyoming), May 13. 1941. _Clethrionomys gapperi uintaensis_ Doutt, Proc. Biol. Soc. Washington, 54:161, Dec. 8. _Type locality._--Paradise Park, 10,050 feet, 45 miles by road northwest Vernal, Uintah County, Utah. _Range._--The Uinta Mountains of northern Utah and southwestern Wyoming.