We collected data on parasitic prevalence and leaf-swallowing behavior of bonobos (Pan paniscus) between August 1998 and April 1999 at the Iyema research site, Lomako Forest, Democratic Republic of Congo. We report the first detailed observations of leaf-swallowing among bonobos and the first record of the behavior at Iyema-Lomako. Bonobo leaf-swallowing closely fits the description of the behavior among chimpanzees. Bonobos ingested leaves of Manniophyton fulvum, as occurs in two chimpanzee populations in Central and Western Africa and among bonobos at Wamba, about 200 km from Iyema-Lomako. All leaf-swallowing occured in the rainy season. In conformity with patterns among Mahale chimpanzees, the prevalence of Oesophagostomum sp. infection in bonobos increased after the onset of the rainy season.