Microevolutionary changes in the body size of the bushy-tailed woodrat (Neotoma cinerea) since the last glacial maximum were estimated from measurements of fecal pellets preserved in paleomiddens from the Great Basin and Colorado Plateau of the United States. The changes closely track regional temperature fluctuations simulated by the Community Climate Model of the National Center for Atmospheric Research and also those estimated from deuterium isotope ratios of plant cellulose recovered from paleomiddens. Body size decreased during periods of climatic warming, as predicted from Bergmann’s rule and from physiological responses to temperature stress. Fossil woodrat middens, by providing detailed temporal sequences of body sizes from many locations, permit precise quantification of responses to climatic change that have occurred in the past and may occur in the future.