_Montana_: Glacier County, August, 6, 1.8; Hill and Chouteau counties combined (Mich), July, 24, 1.5; Malta, Philips County, August, 14, 1.5; Sheridan County, August, 6, 1.5; Fergus County (USBS), August, 5, 2.4; Ravalli County (KU and USBS), August, 12, 2.8; Silver Bow County, August, 7, 3.0; Sweet Grass County (Mich), June and July, 7, 2.7; Park County, August, 10, 2.6. _Idaho_: Pocatello and vicinity, November and December, 5, 3.4. _Wyoming_: Park County, August, 6, 2.8; Sheridan County, September, 9, 1.2; Johnson County, August, 12, 1.5; Campbell and Crook counties, July, 11, 1.4; Weston County, July, 7, 1.6; Teton County, September, 8, 3.4; Teton County (Mich), June, 17, 3.1; Afton and vicinity, Lincoln County, July, 10, 4.2; Sage, Lincoln County, July, 5, 5.0. _South Dakota_: Pennington County (Chi), June, 14, 2.1; Pennington County (Mich), December and January, 8, 1.1; Walworth County, July, 4, 3.7; Buffalo County, July, 6, 3.2. _Colorado_: Loveland and vicinity, Larimer County (KU and USBS), July, 13, 2.8; Boulder County (Chi), September, 34, 2.6; Park County (Denv), March, 8, 1.9; Colorado Springs (ERW), March, April, and May, 5, 2.8; Saguache County (USBS), August, 46, 3.0; Conejos County, June, 4, 3.0; Wray, Yuma County (USBS), 3, 4.7. _Nebraska_: Dundy County, August and November, 14, 4.6. _New Mexico_: Colfax County, June, 8, 3.2. Variation in color is discussed in the accounts of the subspecies concerned. [Illustration: FIGURE 1. Geographic variation in color in _Microtus pennsylvanicus_ in the Rocky Mountains. Paler colors are represented by smaller numbers. Numbers are derived from the series of specimens listed in the text by the method described there. The subspecies that occur in the region studied are as follows: _a. M. p. pullatus_ _b. M. p. insperatus_ _c. M. p. uligocola_ _d. M. p. finitus_ _e. M. p. modestus_ _f. M. p. aztecus_ _g. M. p. drummondi_ _h. M. p. pennsylvanicus_]