Aggression is a behavioral trait that has important implications for many issues in behavioral ecology at both the individual and group level, including variation in individual fitness, mating patterns, social dynamics, and group fitness. Here, we examine the effects of aggression and hyperaggression (the tendency of some males to aggressively attempt to mate with other males or with pairs) on male and female behavior and group and individual mating success in stream water striders (Aquarius remigis). Our results showed that males displaying hyperaggressive behavior play a keystone role in reducing group mating by reducing the activity, aggression, and use of open microhabitats by other males who share a pool with hyperaggressive males; that is, group selection acted against hyperaggressiveness. In contrast, the presence of hyperaggressive males had little detectable effect on the behavior of females. This was likely due to offsetting effects where negative effects of hyperaggressive males on female activity or use of open microhabitats were countered by hyperaggressive males reducing harassment of females by non-hyperaggressive males. On average, hyperaggressive males gained little or no mating benefit from their behavior although there was a trend for the most hyperaggressive males to gain higher mating success than other males in the same pool.