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The purpose of this study was to assess the effluents from a rainbow trout (O. mykiss) farm located in Golestan Province, and their impact on the water quality. Water samples were taken from input and output waters of farm every 20 days during four months. Water quality analysis results showed that rainbow trout farm had a significant impact on temperature, pH, TSS, Dissolved Oxygen (DO), No2, No3, and NH3 concentrations. The amount of dissolved oxygen (DO) in the effluent water of farm decreased compared to the water input significantly, while the values of pH, TSS, NO2, NO3, NH3, increased. According to the results of this study, the water quality and physicochemical factors are influenced by the effluent from rainbow trout farm due to the changes in the environmental conditions and the farm activities.