To study geographic variation in color a method was devised as follows: A single skin (KU 42407, from 1-1/2 miles east of Buckhorn in Weston County, Wyoming) was selected as a representative of the paler mice and arbitrarily given the number 2. A single skin (KU 17491, from 3 miles east of Moran in Teton County, Wyoming) was selected as a representative of the darker mice from the western part of Wyoming and arbitrarily given the number 4. These mice were selected so that they were respectively paler and darker than the estimated average of the total variation within the populations to be studied, but the two mice were not at the extremes of paleness and darkness. Comparisons were based on visual inspection of the dorsal pelage as a whole. Skins were compared with these two mice and given whole numbers from one to five. If paler than the standard for 2, the skin was numbered one; if not distinguishably paler or darker, it was given the number two; if intermediate in color to the standards for 2 and 4 and not definitely more nearly referable to one than the other, it was given the number three; if it resembled the standard for 4, it was numbered four; and if darker, it was given the number five. In this manner skins from a given locality could be evaluated one by one and the results plotted, averaged, and treated statistically. On Figure 1 the average values for color of 32 series are mapped to show the geographic variation of color. The following series of adults are the basis for Figure 1 (abbreviations for collections other than at the University of Kansas are included in parentheses): Each locality is followed by the month (or months) of capture, the number of specimens, and the average value for color.