In this research, controlled changes of rainbow trout muscular composition cultured in brackish water (14.1ppt) and freshwater (4.3ppt) was compared. Some 180 juvenile rainbow trout with a weight of 47.2±0.1g each were released in 6 polyethylene tanks with a capacity of 1.5m³. During the experiment, water temperature was kept constant. Feeding fish was conducted using common Faradaneh food and considering water temperature and fish biomass. After 126 days, fishes attained around 320±20g of weight, were sampled and taken to laboratory and the amount of protein, fat, ash, minerals and moisture in their muscle tissue were measured. Results showed amount of fat and dry matter in brackish water cultured fishes is less than those cultured in freshwater. The average percentage of protein and minerals in brackish freshwater cultured fishes were significantly different and the average percentage of moisture in brackish water cultured fish was higher than those of the freshwater fish
