fa <span style="font-family:times new roman,serif;"><span style="font-size:12.0pt;">Evaluating the effect of dietary soybean oil (S.O) on growth performances of juvenile ornamental fish Severum <em>(Heros severus) </em>was the main objective of this study. To achieve this objective, 462 fish were considered</span></span><span style="font-family:times new roman,serif;"><span style="font-size:12.0pt;"> which </span></span><span style="font-family:times new roman,serif;"><span style="font-size:12.0pt;">were divided into 7 test groups including 6 groups of fishes (with an initial average weight of 0.6 &plusmn; 0.05 g and length of 2.3&plusmn;0.35 cm) fed diets containing 6 levels of S.O (D₁(3), D₂(5), D₃(8), D₄(10), D₅(12), D₆(14)%) and the control group (CG) (receiving an oil-free diet) for 12 weeks with biometry intervals of 15 days. At the end of the test period, growth performance and nutritional parameters were measured. The study showed that the maximum weight was in fish fed diet containing 8% of S.O and also the maximum specific growth rate (SGR) (D₃, <em>p</em><0.05) was obtained for this group. The highest rate of daily weight gain (DWG) was obtained for D₃ which was significantly (<em>p</em><0.05) much more than that for D₄, D₆and D₅ (the lowest increase). The highest length gain (LG) was observed in D₃ which was significantly more (<em>p</em><0.05) in comparison with the other treatments tested. The survival rate was over 80% in all test groups and showed no significant difference (<em>p</em>>0.05). At the end of the period, fish were evaluated for estimated analysis of the tissue. The results</span></span> <span style="font-family:times new roman,serif;"><span style="font-size:12.0pt;">of this study which was focused on the growth and development of ornamental fish <em>H. severus</em> fed with different levels of soybean oil suggest that the optimal level of this type of oil in their diet is 8% based on the growth parameters.</span></span> Soybean oil, lipid, Growth performances, Diet formulation, Severum (Heros Severus) 1030 1042 http://jifro.ir/browse.php?a_code=A-10-1302-2&slc_lang=fa&sid=1 S.E. Vesal 100319475328460022555 100319475328460022555 Yes A.R. Vosooghi 100319475328460022556 100319475328460022556 No