A free-ranging group of Saimiri sciureus was studied in a semi-natural forest habitat in eastern Amazonia, where behaviour patterns were broadly similar to those recorded for the species in the wild. According to focal-animal samples, the monkeys spent the vast majority of their time foraging and feeding, in particular for arthropod prey, which contributed almost half of identified food items. The predation of a small-bodied bat was also observed, although the study animals did not appear to forage systematically for chiropterans in the manner recorded for Saimiri oerstedi.