Aposematic, toxic dendrobatids have received much attention in recent years, resulting in a greater understanding of their biology and evolution; in contrast, data on natural history, social behavior, and systematics are unavailable for most of the approximately 100 species of Colostethus, one of the basal, nontoxic clades of dendrobatids. We studied reproductive and social behavior in Colostethus caeruleodactylus, a species that occurs in igapó forest in the central Amazonian region of Brazil. Reproduction occurs during the rainy season. Males are territorial and can be distinguished from females and juveniles by bright blue coloration of the fingers. Courtship is prolonged and territoriality may serve to prevent interruptions by other males during this critical time. During courtship, which does not include amplexus, males guide females to rolled or folded leaves that serve as nests on the forest floor. Males continue to call and mate while attending nests, which may have more than one clutch. Tadpoles hatch but unlike other species of Colostethus remain in the nest until igapó forests are flooded late in the rainy season, thus providing the larval habitat. This constraint imposed by the delayed formation of the larval habitat may have implications for the structure of the mating system in this species. More information on social behavior of other species of Colostethus coupled with a reliable phylogeny showing relationships among the species will be necessary to interpret the evolution of reproductive behavior and mating systems in this complex group.