We determined life expectancy (to within approximately 3 d) of 1457 prairie voles. Microtus ochrogaster, in a free-living population over a period of 6 yr. During this time the population underwent four cycles of abundance, with peak densities of 160, 250, 350, and 624 individuals per ha. Life expectancy was calculated based on the interactions of season, phase of the population cycle, population density, type of natal social group, and weather conditions 21 and 30 d before and after birth. Life expectancy was greatest for animals born (1) in autumn, (2) during the increase phase of the population cycle, (3) at population densities >100/ha, and (4) into communal groups. The combination of these factors appears to result in a surge in population density in late autumn. The short life expectancy of animals born during the peak and decline phases of the population cycle may accelerate the rate of the decline phase. During periods of high temperatures before birth in summer, life expectancy increased with increased precipitation. There was no such interaction of precipitation and temperature after birth. In both summer and autumn, high precipitation after birth resulted in lower life expectancy, possibly reflecting increased mortality of young forced to leave flooded underground nests.