en aquaculture aquaculture پژوهشي Orginal research papers This study investigated the effect of <em>Sargassum angustifolium</em> and <em>Gracilaria pulvinata</em> extracts on the growth performance, body composition, antioxidant status and gut microbiota of rainbow trout (<em>Oncorhynchus mykiss</em>) fry. For this purpose, 540 fry of rainbow trout (initial weight 0.23&plusmn;0.04 g) were distributed in 12 fiberglass tanks. Fish were fed with experimental diets containing 500 mg kg^-1 of <em>Sargassum</em> extract (SA), 500 mg kg^-1of <em>Gracilaria</em> extract (GL) and 250 mg kg^-<em> Sargassum</em><strong> + </strong>250 mg kg^-1<em> Gracilaria</em> extract (SA+GL). The control diet was a commercial diet without seaweed extract. At the end of experiment growth and feeding performance including final weight, specific growth rate (SGR), weight gain (WG) and feed conversion (FCR) in all treatments were not significantly different (<em>p</em> > 0.05). Whole body protein, ash and moisture contents of fish were not significantly affected by dietary macroalgae extracts (<em>p</em> > 0.05). However, lipid content was significantly lower in fish fed of extract <em>Gracilaria</em> and <em>Sargassum</em> compared to control (<em>p</em> < 0.05). The seaweed extract-fed fish utilized efficiently the body lipid and suppressed body weight loss during starvation. No significant difference (<em>p</em> > 0.05) in serum total protein, creatinine alanine transaminase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP) and blood superoxide dismutase (SOD) activity were observed in all treatments. Whereas serum albumin, glutathione peroxidase (GPX) and catalase specific activity were higher in SA, GL and &nbsp;SA+GL treatments compared to the control group (<em>p</em> < 0.05). ACH50 level was significantly (<em>p</em> > 0.05) increased in GL and SA+GL treatments as compared SA and control group. The addition of<em> Sargassum</em> and <em>Gracilaria</em> extracts to feed did not affecte total bacterial population but increased lactobacillus bacteria levels (<em>p</em> < 0.05) in the intestine. These results revealed the potential antioxidant enhancing and health-promoting effects of dietary<em> Sargassum angustifolium</em> and <em>Gracilaria pulvinata</em> extracts.<br> <br> &nbsp; Macroalgae extract, Immunostimulant, Bacterial population, Antioxidant status, Rainbow trout 926 944 http://jifro.ir/browse.php?a_code=A-10-3232-1&slc_lang=en&sid=1 M. Yazdanpanah mehdi63978@gmail.com 100319475328460041003 100319475328460041003 No Department of Fisheries, Faculty of Agriculture and Natural Resources, Persian Gulf University, Bushehr, Iran E. Sotoudeh e.sotoudeh@yahoo.com 100319475328460041004 100319475328460041004 Yes Department of Fisheries, Faculty of Agriculture and Natural Resources, Persian Gulf University, Bushehr, Iran H. Mansouri Taee mansouritaee@gmail.com 100319475328460041005 100319475328460041005 No 2Lorestan Fisheries Adminestration, Khoramabad, Iran H. Habibi dr.h.habibi@gmail.com 100319475328460041006 100319475328460041006 No Department of Animal Husbandry, Faculty of Agriculture and Natural Resources, Persian Gulf University, Bushehr, Iran