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Sexual size dimorphism (SSD), or the difference in body size between males and females, is common in almost all taxa of animals and is usually assumed to be adaptive. Darwin's two main mechanisms for the evolution of SSD, sexual selection for larger males and fecundity selection for larger females, have often been demonstrated. However, males and females frequently share both genes and environment and more recent papers have noted that males and females must experience differences in lifetime selection on body size for SSD to be maintained. Over two generations I examined lifetime selection acting on adult body size (total length) in a common insect where females are larger than males, the waterstrider Aquarius remigis. Both fecundity selection for larger females and sexual selection for larger males are occurring in this species and both selective forces appear to target specific components of body size rather than total length; sexual selection targeting male genital length and fecundity selection targeting female abdomen length. While body size did not appear to influence adult prereproductive survival, longevity during the reproductive season was negatively related to body size for both sexes. When the opposing selection of reproductive success and reproductive longevity are combined, both males and females have an intermediate optimum body size. A remarkable result of this stabilizing selection was that the optimum size of males was smaller than that of females. I also examined the repeatability of reproductive success in both sexes and the trade-off between egg size and egg number. Finally, estimates of the quantitative genetic basis of the traits examined indicate that both male and female body size, and components of body size, are heritable and can respond to the selection detected. Components of body size in A. remigis are variable in both the degree and direction of sexual dimorphism and the genetic analysis indicates partial isolation of dimorphic and