To assess the impact of a sewage plant on fish, brown trout Salmo trutta were kept in two cages for 55 days in a moderately polluted river upstream of a sewage plant. In one of the cages, undiluted treated waste water of the sewage plant (WWE) was added at an average concentration of 5%, whereas the other cage received river water (R) only. A high mortality occurred in the WWE group. In comparison to control trout held in tap water, the skin structure and ultrastructure were altered clearly in both groups exposed to river water, including necrosis, apoptosis, decreased number of mucous cells, decrease in epidermal thickness, invasion of leucocytes, extension of melanocytes into the epidermis, being gradually more prominent in the WWE group. The most obvious difference between the two exposed groups was found in structure, size and electron density of the secretory vesicles of the filament cells. This and the observed vacuolation of Golgi saccules are indicative for disturbances in the secretory pathway of the filament cells. Certain toxins were suspected to cause the decompaction of myelin sheaths demonstrated in both groups. Reasons for the rather minor overall differences between the exposed groups are discussed. The extremely high mortality rate in the WWE group supports the importance of reducing the load of pollutants in the effluent of the waste-water management plant.