en fish disease fish disease پژوهشي Orginal research papers The aim of this study was to investigate the effect of red pepper (<em>Capsicum annuum</em>) extract on growth, blood, biochemical and non-specific immune parameters of 360 rainbow trout (<em>Oncorhynchus mykiss</em>) juveniles with average weight of 9.38&plusmn;0.01 g. The fish were fed with a commercial diet containing 0, 0.25, 0.5 and 1% of <em>C. annuum</em> extract in 12 tanks (300L) with three replicates (30 fish per tank) for 30 days. Then, <em>Yersinia ruckeri</em> was injected by intraperitoneal to evaluate fish resistance. At the end of fish feeding period, growth and survival, as well as blood and immune parameters were assessed. The highest level of weight gain (%) and SGR and also the least level of FCR were observed in fishes fed by 0.5% red pepper extract (<em>p</em><0.05). Hematological parameters (RBC, WBC and hemoglobin) significantly increased in fish fed with 0.25 and 0.5% of <em>C. annuum</em> extract for 30 days and after challenging with<em> Y. ruckeri</em> as compared to the control group. Lysozyme and complement contents indicated significant increases in all treatments fed with red pepper extract after bacterial infection (<em>p</em><0.05). Generally, it can be concluded that rainbow trout fed with 0.5 % red pepper extract for a month could gain more weight and enhance immune responses against infectious diseases. Red pepper extract, Immunity, Rainbow trout, Yersinia ruckeri, Blood, Growth 1573 1588 http://jifro.ir/browse.php?a_code=A-10-1784-1&slc_lang=en&sid=1 F. Firouzbakhsh f.firouzbakhsh@yahoo.com 100319475328460042715 100319475328460042715 Yes Sari Agricultural Sciences and Natural Resources University S. Haghparast sarah_haghparast@yahoo.com 100319475328460042716 100319475328460042716 No Sari Agricultural Sciences and Natural Resources University M.R. Memarzadeh m.r.memarzadeh63@gmail.com 100319475328460042717 100319475328460042717 No Barij Medicinal Plants Research Center