A feeding trial was conducted to examine the effects of dietary supplementation of betaine on growth performance, haematological-biochemical parameters, antioxidant status and digestive enzyme activities of juvenile Caspian trout, Salmo trutta. A feeding trial was conducted to examine the effects of dietary betaine on growth performance, haematological-immunological parameters, antioxidant defence system and digestive enzyme activities of juvenile Caspian trout, Salmo trutta. For this purpose, 180 juveniles (9.7 ± 0.59 g-total length, 9.35 ± 0.48 cm) were fed four experimental diets as follows: plant-protein basal diet, the basal diets with 1.4 and 2.8% betaine supplementation or a fishmeal-based diet (control). Triplicate groups were considered for each treatment and feeding trial lasted for 77 days. The results revealed that weight gain (WG) ratio, specific growth rate (SGR) and protein efficiency ratio (PER) were significantly higher in fish fed the betaine-supplemented diet (1.4 & 2.8%) and control diet than in the group fed the basal one. Lysozyme activity and total serum immunoglobulin of those fish fed the basal diet with 2.8% betaine supplementation or the control diet were also found significantly higher than those in fish fed the basal diet.Moreover, significantly lower (p < .05) plasma total cholesterol, glucose, high-density aspartate transaminase (AST), alanine transaminase (ALT), alkaline phosphatase (ALP) and lactate dehydrogenase (LDH) activities were observed in the basal diet, but those were improved as supplemented with 1.4 and 2.8% betaine. The study of antioxidant enzyme activities revealed a rising trend in superoxide dismutase (SOD), glutathione S-transferase (GST) and catalase (CAT) in 2.8% betaine induced by the plant-proteinbased diet (p < .05). Furthermore, digestive enzyme activities including lipase and amylase had augmented in fish receiving betaine-supplemented diets. Results indicated that, in diets based on plant proteins that often have low palatability, 1.4% of betaine supplementation might improve feed acceptability, growth performance, humoral immune responses, antioxidant defence and digestive activity in Caspian trout.