During the course of our epizootiological surveys on neoplasms in coastal fishes, an estuarine area (station A) was unique in the high incidences of chro-matophoroma in the croaker, Nibea mitsukurii, and of skin pigment cell hyperplasia in the sea catfish, Plotosus anguillaris, respectively. The following results of our studies suggest that the pigment cell disorders in these species may be useful biomakers to monitor the coastal water pollution by carcinogens: