Red-sided garter snakes (Thamnophis sirtalis parietalis) court and mate in early spring around large communal overwintering dens in central Manitoba. Emerging females are immediately covered by dozens or hundreds of vigorously-courting males, potentially imposing significant costs to the female. By manipulating numbers of courting males (both directly and by applying anticourtship pheromones), we quantified the degree to which female dispersal from the den is hindered by courtship. Courted females dispersed only about half as fast as did solitary females. Blood lactate levels were higher in mating than in courting or noncourting snakes of both sexes; the high levels of lactate in mating females support the idea that courtship is physiologically stressful to these animals, perhaps via constraints to female respiration. In arena trials, females that were exercised to exhaustion before courtship mated with smaller males than did control females. The spatial distribution of snakes around the den exhibits substantial heterogeneity, with densities often varying markedly between adjacent areas. Arena trials mimicking this heterogeneity showed that unmated females avoided parts of the enclosure containing scent cues from males. Our data support the hypothesis that courtship in T. s. parietalis confers significant costs to females, and that female behaviors have evolved to reduce those costs.