We examined the reproductive biology of 522 female and 330 male dusky dolphins, Lagenorhynchus obscurus, killed in Peruvian coastal fisheries between 1985 and 1990. Most births occurred in late winter (August, September, and October), although a few newborns were collected as late as February. Growth rate of fetuses was 0.261 cm/day; young were born at a mean length of 91 cm and mass of 9.6 kg. Gestation lasted for 12.9 months, followed by 12.0 months of lactation and a resting period of 3.7 months. The size of adult testes increased, reaching a maximum in September and October, in synchrony with the peak period of conception. The largest single-testis mass (with epididymis) was 5,120 g, and the maximum ratio of testis:body mass in mature males was 0.085, among the highest of any mammal. The large testes, relative sexual monomorphism, and apparent lack of aggressive behavior between males suggests a promiscuous mating system and sperm competition in this species.