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A four-week trial was conducted to evaluate the effects of different levels of dietary Betaine on growth, survival and resistance to salinity (35ppt), temperature (24 C), hypoxia and high and low pH stresses in rainbow trout (Oncorhynchus mykiss). Four levels of Betaine as 0 (control), 0.4% (B0.4), 0.8% (B0.8) and 1.2% (B1.2) were used with three replicates. Forty fish (0.67±0.15g) were stocked in each replicate and fed at 4-4.5% of body weight, five times daily. Results showed that there were significant differences in growth, survival, NPU, PER and FCR between B0.4, B1.2 and control diet (P<0.05). Carcass analysis showed that there was higher protein in the carcass of fish fed with diet B0.4 compared to control and B1.2 diet. There were no significant differences in carcass protein content between B0.4 and B0.8. Also, no significant differences were found in carcass crude fat, ash and dry matter among different treatments (P>0.05). Resistances to high and low pH (10.8 & 3.8) and to high temperature (24 C) were significantly better in fish fed with Betaine containing diets compared to the control diet. Resistance to high salinity in B0.4 was significantly better than the control diet and resistance to hypoxia in diet B0.8 was significantly better than the control (P<0.05). The results showed that dietary supplementation of Betaine (0.4%) have positive effects on growth factors, survival, body composition and resistance to environmental stresses in rainbow trout. *Corresponding author