ABSTRACT. – Domestic wastewaters of Tunceli, are directly discharged to Munzur and Pulumur Rivers without any pre-purification process. The purpose of this study is to reveal the size of pollution caused by domestic wastewaters together with natural contamination by the drainage area of the Pulumur and Munzur Rivers and to monitor physiological changes in fish captured from Uzuncayir Dam Lake. In fish samples, superoxide dismutase, catalase, glutathione peroxidase activities and levels of glutathione and malondialdehyde were determined. Our studies showed that the antioxidant system of Capoeta umbla could be used as biomarker of exposure to aquatic pollutants in Uzuncayir Dam Lake.