Frequencies of external and grossly visible liver tumors were high in brown bullheads Ictalurus nebulosus from Black River, an industrialized Lake Erie tributary. Skin and liver tumors were lacking in brown bullheads from Buckeye Lake, a reference site, and the incidence of lip tumors was low (<2%). Liver tumors, primarily cholangiocarcinomas, increased significantly (P < 0.05) with age in Black River brown bullheads, occurring in 28–44% of the 4-year-old fish collected in 1980–1982. External tumors did not increase consistently with age. Within year classes, the occurrence of grossly observable liver tumors was higher in fall than in spring. Liver tumor incidence was not correlated with sex. Organic contaminants were higher in brown bullheads from the Black River than in those from Buckeye Lake. Polynuclear aromatic hydrocarbons were particularly high, including carcinogens such as benz[a]anthracene and benzo[a]pyrene (B[a]P). Limited electron microscopy of cholangiocarcinomas provided no evidence of virus. Since brown bullhead have the ability to hepatically activate B[a]P, and since B[a]P has been shown to induce cancer in fish, the hypothesis of a chemical etiology for the tumors in the Black River brown bullheads best fits the available data.