Male northern water snakes (Nerodia sipedon) have high variance in reproductive success relative to females. We used DNA-based paternity analyses from a 3-year study of two marsh populations of water snakes to investigate the factors that contribute to variation in male success. Male traits investigated included body size, condition, tail length, home range size, activity during the mating season, and genetic profile (genetic similarity to females, heterozygosity, and genetic variability [d2]). We successfully assigned > 80% of offspring to sires from a sample of 811 offspring from 45 litters. Male reproductive success did not vary significantly with body size, tail length, condition, home range size, or the number of microsatellite loci at which males were heterozygous, nor with other features of their genetic profiles. However, we found evidence of positive assortative mating by size in the marsh in which receptive females were not spatially clumped. Also, males that were most active during the mating season were more successful, particularly where females were not clumped. We failed to find evidence of selection acting on male size through variance in reproductive success, indicating that sexual selection does not have an important influence on sexual size dimorphism in this species (males are smaller than females). We propose that males are smaller than females because the lack of advantage to large size allows males to adopt a low-energy, low-growth strategy that reduces their risk of predation outside the mating season.