This study was conducted at the Hanna Reservoir located in the southwest of Isfahan Province. According to this study, chemical analyses were conducted to evaluate the magnitude and variations of COD, BOD5, O2, NH3+ and NO2-concentrations in the Hanna Reservoir. In the headwaters the magnitude values of BOD5, COD and early morning oxygen minima were quite identical to the recommended optimum levels. However the concentrations of :::union:::ized ammonia in winter and spring and the concentration of nitrite in summer and autumn exceeded the allowable levels cited in different literature and might have exerted some growth retarding or lethal effects on the fish population. Water quality, zooplankton, benthos and sediments were monitored at sites of 20 m depths to determine the effects of rainbow trout cage culture on the lake environment. Oxygen depletion occurred in the vicinity of the farms due to respiration of the farmed fish. Zooplankton had stronger nutritional value in all six ecosystems of the Hanna Reservoir, and in seasons during which fish feed on these nutritional sources, breeding occurs with higher efficiency. Benthic invertebrates were rare at all six sites of the lake. The sediments below the cages had a pH and organic matter comparable to areas of the lake receiving the natural input of allochthonous material.