Information on the phenotypic correlates of male reproductive success can provide important insights into the operation of sexual selection, and the nature of evolutionary forces on phenotypic traits such as male body size. We combined results from a long-term mark-recapture field study with genetic analyses for identifying paternity of offspring to quantify male reproductive success in a colubrid snake species Final acceptance 30 September 2008 from tropical Australia. Because previous work has shown that male slatey-grey snakes, Stegonotus cucullatus, attain larger body sizes than do conspecific females, we predicted that larger males would have higher reproductive success. Our paternity assignments of 219 offspring (24 clutches) supported this prediction: larger males fathered more offspring, not because they obtained more matings, but because they fathered a higher proportion of offspring within the clutches to which they did contribute. Multiple paternity was common (mean of 2.3 fathers per clutch, range 1-5). Our results demonstrate the utility of molecular approaches to clarify mating systems in field populations of snakes, and suggest that the evolution of extreme male-biased sexual size dimorphism in this species is attributable to enhanced reproductive success afforded by larger body size in males.