Abstract A croaker (Nibea mitsukurii) has a high incidence of the skin melanoma, chromatophoroma, in a Pacific coastal area in Japan. A sea catfish (Plotosus anguillaris) bearing skin melanosis is also found in the same area. For elucidation of a correlation between these pigment cell neoplasms of the skin and environmental contaminants, an epidemiological survey was conducted to determine the distribution and prevalence of tumor-bearing fish. Based upon observations of a high prevalence of skin neoplasms near the discharge point for kraft pulp mills, experiments were conducted to determine the neoplastic induction efficiency of the effluent on the croaker and sea catfish species. Isolation and identification of mutagens in effluent extracts were carried out using the Ames test, followed by mass spectral analysis of mutagenic fractions. The effluent induced a chromatophoroma on one croaker of the 100 tested, and it induced pigment cell hyperplasia on 70 to 100% of the sea catfish. These skin neoplasms were grossly similar to those observed in the field. Five chloroacetones were identified from the Ames-positive fractions of the effluent, and tetrachlorocyclopentene-1,3-dione and two α-dicarbonyl compounds were also detected as mutagens. The above experiments indicate that the mutagenic contaminants found in kraft mill effluent may play an important role in the induction of skin neoplastic disease in fish.