The voles Microtus montanus, M. longicaudus, M. mexicanus, and M. pennsylvanicus occur in New Mexico and all reach their distributional limits in the general region. The four species differ in degree of ecological and geographic restriction and in amount of geographic variation. Degree of geographic variation seems to be positively correlated with degree of restriction and isolation. A possible case of character displacement resulting from competition of closely related species is discussed. Historical biogeographic factors possibly leading to the present situation are considered.