Observations of breeding activities of male and female crayfish ( Orconectes rusticus) in a natural stream population during the explosive spring breeding period, and supplemental laboratory observations, indicate a promiscuous mating system characterized by intense intermale aggression. Males wandered extensively over the substrate, fought frequently with each other, and attempted to disrupt copulating pairs. Larger males (as distinguished by large chela size) more frequently initiated and won aggressive encounters and interrupted copulating pairs more successfully than smaller males, but were subjected to greater predation. Intense intermale aggression ceased when females sequestered themselves under rocks in the stream or were removed from the laboratory population. Brief resistance by females to approaching males, along with sequestering behaviour, suggests that female choice may also occur.