We used data collected over 3 years at two study sites to quantify the rates and consequences of multiple paternity and to determine the opportunity for selection on male and female northern water snakes (Nerodia sipedon). We sampled litters from 45 females that gave birth to 811 offspring. Using eight microsatellite DNA loci (probability of exclusion of nonparental males > 0.99), we assigned paternity to 93% of neonates from one study population and 69% of neonates from the other population. Observations of participation in mating aggregations predicted individual reproductive success poorly for two reasons. First, males regularly courted nonreproductive females. Second, more than half of all sexually mature males obtained no reproductive success each year, despite the fact that many of them participated in mating aggregations. The number of sires per litter ranged from one to five, with 58% of all litters sired by more than one male. Multiple paternity increased with female size, apparently both because bigger females mated with more males and because the larger litters of big females provide paternity opportunities to more males. Multiple paternity was also more prevalent in years with shorter mating seasons. We detected no advantage to multiple paternity in reducing either the number of unfertilized ovules or stillborn young. Despite the majority of males siring no young each year, some males fathered young with as many as three different females in one year. Male reproductive success increased by more than 10 offspring for each additional mate, whereas female success increased by fewer than 2 offspring for each additional mate. The opportunity for sexual selection was more than five times higher in males than females.