There is currently little information on the biology of anurans of the genus Crossodactylus (Leptodactylidae, Hylodinae), which comprise small diurnal frogs that live associated with streams in areas of Atlantic Rainforest. In this study, we analyzed some aspects of the natural history of Crossodactylus aeneus in an area of Atlantic forest in the state of Rio de Janeiro, southeastern Brazil. A total of 37 individuals of C. aeneus were collected during the study, all of them associated with streams inside the forest. Most of the individuals (60%) were found on rocks. Most of the animals were collected during the day, although some were active at night. The diet of C. aeneus was composed of various arthropods, with beetles, ants, dipterans and insect larvae being the predominant items. Of the 21 collected females, eleven contained 70–127 unpigmented vitelogenic eggs.