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The present study was designed to evaluate the possible effect of dietary Origanumvulgare extract as a non-specific immunostimulating agent in rainbow trout (Oncorhynchusmykiss). Six hundred (600) averages mean weight 13±0.05 g rainbow trout (Oncorhynchusmykiss) were randomly allocated into two groups including placebo-treated group (control), and Origanumvulgare extract-treated group, each of three replicates. The fishes were hand-fed once a day with diet medicated placebo or Origanumvulgare extract (OE) at a rate of 1% of feed weight in the first feeding for 8 weeks. At the end of every two weeks 24 hrs after feeding, fish were bled from caudal vein and blood samples were analyzed for some of hematological and immunological parameters. The results showed that serum total protein, albumin and globulin, respiratory burst activity, phagocytic activity and serum lysozyme activity vary among the two treatment groups which were found to be higher in OE-treated group (P< 0.05). It was concluded that supplementation of OE at a rate of 1% registered higher immunological responses. Therefore, dietary inclusion of OE could improve non-specific immune responses in rainbow trout. Future studies to determine optimal herb mixtures and dietary levels should be conducted.