_Specimens examined._--Total 256. _Wyoming_: YELLOWSTONE NATIONAL PARK: Mammoth Hot Springs, 17 (USNM). PARK CO.: 3-1/5 mi. E, 3/5 mi. S Cody, 5020 ft., 15. TETON CO.: _Whetstone Creek_, 4 (Mich); 5 mi. N Moran, 13 (Mich); _Moran and environs_ (_4 localities within 4 miles of Moran_), 6200 ft., 54; Teton Park, Trappers Lake, 3 (Mich); _Teton Park, Jenny Lake_, 1 (Mich); _Teton Park, String Lake_, 1 (Mich); Sheep Creek, 1 (Mich); _Jackson and environs_, 115 (Mich 113, USBS 1). SUBLETTE CO.: 34 mi. N, 4 mi. W Pinedale, 7950 ft., 2; Kendall, 5 (Mich). LINCOLN CO.: 9-1/2 mi. N, 2 mi. W Afton, 6100 ft., 1; _9 mi. N, 2 mi. W Afton_, 3; _7 mi. N, 1 mi. W Afton_, 11; _15 mi. N, 3 mi. E Sage_, 6100 ft., 1; _12 mi. N, 2 mi. E Sage_, 4; 6 mi. N, 2 mi. E Sage, 2. Microtus pennsylvanicus insperatus (Allen) _Arvicola insperatus_ Allen, Bull. Amer. Mus. Nat. Hist., 6:347, December 7, 1894. _Microtus pennsylvanicus insperatus_, Anderson, Canadian Field-Nat., 57:92, October 17, 1943. _Microtus pennsylvanicus wahema_ Bailey, Jour. Mamm., 1:72, March 2, 1920. _Type._--Adult male, skin and skull, number 8105/6731 American Museum of Natural History, obtained by W. W. Granger, at Custer, Black Hills, South Dakota, August 9, 1894. _Range._--Western South Dakota, southwestern North Dakota, eastern Montana, southern Alberta and Saskatchewan, eastern Montana, and northeastern Wyoming. _Comparisons._--_Microtus pennsylvanicus insperatus_ is paler than any adjacent subspecies. It has been compared with _pullatus_ in the preceding account. Bailey's studies of _M. p. wahema_ [=_insperatus_] and his comparisons with _M. p. pennsylvanicus_ and _M. p. drummondi_ to the east and north, in North Dakota (1920, 1927), are the basis for the northern and eastern boundaries of the range of _insperatus_ in Figure 1. Comparison of _insperatus_ with _finitus_ to the south is made in the account of the latter. TABLE 1. AVERAGE MEASUREMENTS (IN MILLIMETERS) OF ADULT MICROTUS PENNSYLVANICUS