Influence of cage fish farming on water quality in the eutrophic Nechranice Reservoir was assessed through its contribution to the balance of eutrophication-effective forms of phosphorus (PO4-P) entering the reservoir and through measuring impacts on sediment around farming cages. The study showed that although the contribution of cage fish farming is relatively small (max. in tenths to a few percents of the total PO4-P load) its effect on water quality can not be neglected, because it causes direct pollution and, under certain hydrologic and climatic situations contributes significantly to the eutrophication and to critical deterioration of water quality for recreation and bathing. On the other hand, it is indisputable that the main source of eutrophication of Nechranice Reservoir are discharges of municipal wastewater in the catchment.