An 8-week feeding experiment was conducted to investigate the inclusion potential of five levels of Calotropis persica seed powder -CSP- -0, 10, 20, 30, 40 and 50 g kg-1 of the basal diet- in rainbow trout -Oncorhynchus mykiss- diet, in a completely randomized design. To perform the test, 3600 fries -11.5±3.64 g- were treated for 56 days. According to the results, the increase in milkweed seed powder up to 40 g kg-1, resulted in a significant increase in specific growth rate and intestinal lactic acid bacteria count compared to the control -p<0.05-. The best results of survival rate, feed conversion ratio, hepatosomatic and gastro somatic indices were achieved in the treatments receiving 20, 30, 40, and 50 g kg-1 CSP -p<0.05-. Antibacterial activity of skin mucus, lysozyme and alkaline phosphatase showed the highest level in the 40 g kg-1 treatment -p<0.05-. Based on the results, the inclusion of 40 g kg-1 milkweed seed powder caused positive health effects and could be a suitable herbal feed additive in the rainbow trout diet.

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