The objective of this study was to assess the environmental impact of one of the biggest flow-through rainbow trout farms located in South-Western Turkey between February and December 2006. The feeding water is obtained from the Yuvarlackay Stream (Mugla, Turkey) at a mean flow rate of 2.86 Litre s(-1) per ton of the annual fish production (700 t year(-1)) with drum Filtration used for the effluent treatment of the farm. Bi-monthly sampling was conducted from the inlet and discharge water (before and after treatment) at the same hour of each sampling day No significant differences in total nitrogen (TN), total phosphorus (TP), biochemical oxygen demand (BOD(5)), and chemical oxygen demand (COD) were found in the effluent, although these parameters were found to have increased in the discharged water. The dissolved oxygen (DO) and pH of the effluene water were found to be significantly affected by the fish farming activities. The effluent treatment efficiencies were found to be 7.77% for TP, 8.67% for TN, 16.58% for BOD5, and 13.16% for COD, respectively The annual TP load of the trout farm after effluent treatment was found to be 5.68 kg P per ton of fish produced, and 5.30 kg P per ton of feed used, respectively. The average TP load without any effluent treatment determined via chemical and biological methods was 7.66 and 10.46 kg P per ton of fish produced, respectively.