We studied patterns of and factors influencing abundance of hispid cotton rats (Sigmodon hispidus) in tallgrass prairie habitats from autumn 1981 to spring 2001 at the Konza Prairie Biological Station, Kansas. Abundance of cotton rats was low during autumn (X̄ = 0.57 individuals/trapline) and extremely low in spring (X̄ = 0.04 individuals/trapline). Autumn abundance varied widely in 20 years (range: 0.00–2.50 individuals/trapline) and among 14 sites (0.05–1.35 individuals/trapline). Spring prairie fires had a positive influence on cotton rats during the 1st (X̄ = 0.8 individuals/trapline) and 2nd autumns (X̄ = 1.0 individuals/trapline), but not during the 3rd or later autumns (X̄ = 0.2 individuals/trapline). Cotton rats were associated strongly with lowland prairie relative to breaks and upland prairie. Autumn abundance was correlated positively with previous winter (December–February) average maximum temperature, but was not correlated with previous winter precipitation, summer (June–August) average maximum temperature, summer precipitation, or aboveground net primary productivity. Although previous winter maximum temperature accounted for only 32% of interautumn variation in abundance, winter harshness appears to be the major factor driving temporal variation in autumn abundance of hispid cotton rats.