en aquaculture aquaculture پژوهشي Orginal research papers In the current study, the effect of dietary supplementation of poly-&beta;-hydroxybutyrate (PHB) on growth performance, digestive enzymes activity, body composition, non-specific immune response and diseases resistance of rainbow trout fingerlings were investigated. Five hundred and fifty-two healthy fingerlings (8.2&plusmn;1.0 g) were randomly distributed in 12 tanks (150 L) at a density of 46 fish/ tank and fed different levels of dietary PHB (0, 0.5, 1 and 2%) for 70 days. At the end of the experiment, the results showed that PHB affected growth performance and the fish fed 0.5 and 1% PHB diets had a significant higher weight gain and specific growth rate (SGR) than the control (<em>p</em><0.05). The specific activity for total protease and amylase in 1% PHB was significantly higher than that of both the control and 2% PHB (<em>p</em><0.05). In addition, PHB significantly affected the muscle fatty acids profile but did not change protein and lipid content. Also, our results indicated that 1 and 2% PHB markedly decreased cumulative mortality of the fingerlings challenged by <em>Yersinia ruckeri</em> infection. Overall, the results of this experiment revealed that PHB acts as a growth promoter and immunostimulator in rainbow trout fingerlings and the optimal PHB level in trout diet was estimated at 1% PHB. Poly-β-hydroxybutyrate, Growth promoter, Rainbow trout, Immune response, Yersinia ruckeri 847 865 http://jifro.ir/browse.php?a_code=A-10-2559-1&slc_lang=en&sid=1 E.H. Najdegerami e.gerami55@gmail.com 100319475328460033944 100319475328460033944 Yes Department of Biology, Facultu of Science, Urmia University, Urmia, Iran