Prevalence of diseases in cultural systems is the most important problem in aquaculture. Usually, antibiotics are used to control fish disease as well as stimulate and promote growth and efficiency of diets. Because of some disadvantages of antibiotic usage, such as development of antibiotic-resistant microorganisms and efficacy reduction of antibiotics, using natural immunostimulants is to be considered. It is well known that not only the innate immune system in fish can be triggered by these materials, but also they have beneficial effects on growth, FCR and quality of body composition. Garlic ( Allium sativum ) is one of these materials that have several beneficial effects. So, to evaluate the effects of garlic on growth parameters (including final body weight, SGR, CF, WGP, FCR, HI, PER and PPV), chemical body composition (including protein, moisture and oil), biochemical factors (including TP, Alb, Glb, Alb:Glb, Glc, CHOL, TG, HDL, LDL, GOT and GPT), immune activity (lysosyme activity) and flesh quality of rainbow trout ( Oncorhynchus mykiss ) fingerlings, 360 fish were divided equally among six groups, each comprising three replicates. Group 1 was the control (fed on base diet), groups 2,3,4,5 and 6 were fed on garlic supplemented diet containing 10, 20, 30, 40 and 50 g/kg diet for 60 days, respectively. The results showed a significant increase in body protein among all garlic groups (except group 2) and body oil was significantly decreased compared to control group ( p ? 0.05). Statistically significant increase was observed in final body weight of the control group compared to the garlic groups, whereas WGP and SGR showed a significant decrease just in group 6 ( p ? 0.05) compared to the control group. There were no significant changes in CF, FCR and HI of all groups. Total protein, Albumin and globulin was higher in treatment groups than in the control as it showed the highest amount in group 3 and the lowest level in control group ( p ? 0.05). There was a significant increase in Glb content in group 3, 4 and 5. along with increasing the level of garlic in the diet. Garlic treatment caused cholesterol and triglyceride depletion in group 5 and 6 accompanied with a significant elevation in HDL in group 3, 4 and 5 ( p ? 0.05). Lysozyme activity was significantly enhanced in 40 g/kg diet garlic treated group compared to the control group. Evaluating organoleptic characteristics, including color, odour, taste and hardness, suggested that these parameters (especially taste) had improved by increasing garlic level to 40 g/kg diet .
