I tested the circadian activity rhythm of field-captured prairie voles, Microtus ochrogaster, in experimental laboratory chambers. Microtus ochrogaster displayed a circadian rhythm with major activity during the dark period, whether in a normal or reversed photoperiod. When in constant dark and constant light, there was no distinct circadian rhythm; activity was greater when in constant dark than when in constant light. An activity rhythm with little activity during the daylight hours presumably would reduce risk of predation from diurnal avian predators. I propose such an activity pattern contributes to the ability of M. ochrogaster to achieve high population densities in habitats with sparse vegetative cover.