The presence of carcinogenic and mutagenic chemical(s) in the effluent of a wastewater treatment plant was indicated by papillomas developing on caged black bullheads (Ictaluraus melas), hepatic enzyme induction in exposed fish, and Ames test mutagenicity of organic extracts of the wastewater. Although virus-like particles have been reported in papillomas of several other fish species, no evidence was obtained for the presence of viruses in the black bullhead papillomas. Mutagenic and carcinogenic chemicals were not identified in the wastewater, but chlorination was implicated as a factor contributing to the induction of the papillomas.