This study aimed at evaluating the effects of a probiotic additive in productive performance and physiological parameters for Rhamdia quelen. The fish were fed diets containing increasing levels of the additive (0.15; 0.30; 0.45; and 0.60 g/kg of feed), which consisted of a blend of Bacillus cereus and B. subtilis, both at the concentration of 4 × 10 11 CFU). Productive performance parameters were higher in fish fed diets containing 0.60 g/kg of the probiotics. Both the proximate composition and carcass yield were affected when 0.30 and 0.60 g/kg of the additive was included, whilst serum biochemical parameters were higher in fish fed diets containing 0.60 g/kg of the additive. An increase in the hepatic oxidative stress activity was found when 0.30 g/kg was supplied. The development of intestinal villi was influenced by the inclusion of the additives, as well as the activity of mucous-secreting cells in the tissue. Hepatocyte's growth was differential in fish fed diets containing 0.30 g/kg and other tissue alterations were found using 0.60 g/kg of the additive. The inclusion of 0.60 g/kg of the probiotic in diets for R. quelen enhances its growth performance and physiological status. K E Y W O R D S biotechnology, fish farming, immunomodulatory, native species, non-pathogenic bacteria | 455 LINS-RODIGUES Et aL. by improving the activity of digestive enzymes, besides boosting the animals' immune system (Azevedo et al., 2016; Nakandakare et al., 2013; Wild et al., 2014). In addition, B. subtilis has known bactericidal and fungicidal effects, as it may increase the immune responses of the host (Ramesh & Souissi, 2017). The silver catfish (Rhamdia quelen) became one of the most studied native species by presenting expressive growth, excellent feed conversion, tolerance to handling, high carcass yield and reproductive viability when reared in captivity (Amaral-Junior et al., 2008; Barcellos et al., 2004; Carneiro et al., 2006). In addition, its high acceptance in the consumers market has increasing the number of rearing facilities, which can be performed either in excavated ponds or net-cages, from the initial stages to termination, presenting optimal growth throughout all year, even during periods of low tem