A population of hispid cotton rats, Sigmodon hispidus, inhabiting a remnant grassland in west-central Kansas, near the northern limit of the range of the species, was live-trapped from April of 1965 through February of 1969. Until 1968, the population exhibited a pronounced annual cycle of abundance, varying from most abundant in autumn to least abundant in spring. Annual declines began at the time of normal cessation of breeding with the onset of winter, and were augmented by harsh winter weather. Monthly ecological densities ranged from 0.0-65.5 rats per hectare in the favored habitat. Population turnover was 92 percent complete in six months. In 1968, the population continued to exhibit a typical, seasonal pattern of fluctuation until autumn, but then underwent a "crash" that completely decimated the population. Biotic and environmental factors that might have influenced the crash included normal autumnal cessation of breeding coupled with predation, parasitism, and severe weather conditions. The balance between adverse winter weather and physiological and behavioral adaptations to survive suboptimal weather conditions probably is largely responsible for the location of the northern limit of the geographic range of S. hispidus.