To find out the main cause of the sexual size dimorphism (SSD) in the Cuban treefrog Osteopilus septentrionalis, I studied between April 2002 and April 2004 the breeding behavior and survival of individuals in a population localized in north-western Puerto Rico, West Indies. This species is an explosive breeder whose breeding events lasted one night. Mature females were larger in body size (snout vent-length, SVL) (range 57-107 mm) than mature males (range 41.5-67 mm), and exhibited higher levels of survival (males: 0.72, 95% confidence interval 0.66-0.78; females 0.91, 95% confidence interval 0.88-0.93). Females can increase their reproductive success if they breed with large SVL because large females can lay more eggs than small females. In contrast, the reproductive success of males was not enhanced by large SVL because the mating pattern was random and non-assortative by SVL. Thus, for males it could be better to start to breed with small SVL, and participate in all possible breeding events, which implies high mortality risk. As a consequence, the SSD in O. septentrionalis may be due to fecundity advantages in females, and because males tend to die before reaching larger SVL.