_Microtus ochrogaster_ can be separated from other voles in its geographic range by a combination of several characters. The plantar tubercles usually number five, although a few individuals with six tubercles were found at Lawrence, Kansas. _Microtus pennsylvanicus_, normally with six plantar tubercles, as Bole and Moulthrop (1942:156) pointed out, sometimes has only five. Therefore, the number of plantar tubercles alone is not a certain means for separating _pennsylvanicus_ from _ochrogaster_. The color of the venter of _ochrogaster_ is usually fulvous or cinnamon instead of grayish as in _pennsylvanicus_, but there is variation in this respect too; some prairie voles also have a grayish venter. The shorter tail of _ochrogaster_ will assist in establishing its identity where it occurs with _pennsylvanicus_. The third upper molar has two closed triangles in _ochrogaster_ and usually three in _pennsylvanicus_. The pelage of _ochrogaster_ is coarse whereas _pennsylvanicus_ has fine fur. Prairie voles may be separated from pine mice (_Pitymys nemoralis_ and _P. pinetorum_) with which they are sometimes found, by the larger eyes, less rusty color, and longer tail. The Cooper lemming mouse (_Synaptomys cooperi_) differs from the prairie vole in having the upper incisors grooved, and in possessing a shorter tail which approximates the hind foot in length. Of _Microtus ochrogaster_ from Lawrence, Douglas County, Kansas, average measurements of twenty-five adult males are: total length, 143 (121-167) mm.; tail, 32 (25-42) mm.; hind feet, 20 (17-22) mm.; weight, 43 (38-55) grams. Twenty-five adult females from the same place average: total length, 150 (131-170) mm.; tail, 33 (31-41) mm.; hind foot, 19 (17-21) mm.; weight, 45 (38-58) grams. The prairie vole is found in suitable habitats in the central part of North America. It has been recorded from Edmonton, Alberta, in the northwest (Bailey, 1900:76), southeastward to Chesapeake, Ohio (Bole and Moulthrop, _op. cit._:156), and in the southwest as far as Ft. Reno, Oklahoma (Bailey, _op. cit._:74). See figure 1 showing the known range of _Microtus ochrogaster_. _Microtus ludovicianus_, a close relative of _ochrogaster_, has been taken along the southern part of the boundary between Texas and Louisiana (Lowery, 1943:247).