The mating system of squirrel monkeys (Saimiri oerstedi) in Parque Nacional Corcovado, Costa Rica was studied and used to develop a model to interpret the evolution of seasonal sexual dimorphism in squirrel monkeys (Saimiri spp.). Adult male body weights in captivity and the wild may increase more than 20%, beginning approximately two months prior to and continuing through the annual two month, breeding season. Female inter-troop transfer was common in the study population, but male troop residence was stable. Instances of agression among adult males in the troop, even in sexual contexts, were rare. Reproductively mature males enlarged to varying degrees by the start of the breeding season and cooperated in mobbing females to olfactorily evaluate female, estrous condition. Female mate preference corresponded to a ranking based on relative male enlargement. The largest male obtained 70% of the copulations observed in the 1984 breeding season. Little evidence exists that females typically mate with more than one male during the period of peak receptivity. Seasonal enlargement in males is suggested to be the result, of both male intrasexual competition and female choice.