One of purposes of using medicinal plants in the diet of animals is to improve growth indices and identify growth promoters for organic aquaculture practices. Harmel is a plant having antiinflammatory and anticancer agents. Harmal seeds was used at five inclusion levels (100, 200, 300, 400 and 500 ppm) together with control diet in the isonitrogenous and isoenergetic diets of juvenile rainbow trout (3.1±0.5 g) for 56 days. Growth performance (specific growth rate) and immunological indices (lysozyme and complement) were measured. Results showed that using more than 300 ppm harmal seed decreased the final weight (8.11 ± 0.5 g) compared to control group (10.42 ± 0.8 g). Specific growth rate fish fed the diets containing 400 ppm (2.8%/day) and 500 ppm (2.4%/day) of harmal seed decreased significantly (P≤0.05) compared to control diet (4.1%). Immunological indices including lysozyme and complement levels of the serum of fish fed the diet containing 400 ppm harmal increased (8.2 µg/ml and 10.1 unit/ml, respectively) compared to control diet (6.2 µg/ml and 8.0 unit/ml, respectively). Based on the results, using harmal seed at 300 ppm in the diet of rainbow trout was possible Knowing neutraceutical and functional properties of native plants is an important area in fish nutrition in near future

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