the aim of this study was to evaluate the effects of fish oil acidity on the growth of pacu (piaractus mesopotamicus) juveniles. two hundred juveniles with 10.53±0.65 g of weight and 8.74±0.13 cm of total length, were distributed in a totally randomized design in 20, 0.15 m3 mesh-made tanks placed in a 25 m3 holding tank. extruded commercial diets sprayed with soybean oil (control) with acidity index of 0.24 and fish oil with acidity index of 1.48, 6.40 and 9.85 were used. the fish were fed four times a day until apparent satiety. productive performance, centesimal composition and blood biochemistry were analysed. at the end of the feeding period none of the analyzed parameters was affected by the use of acidified fish oil. therefore, fish oil may be supplied to the pacu juveniles regardless of their ph.