ABSTRACT:The study assessed the effect of phytase on the digestibility of alternative plant feed ingredients Brazil nut (Bertholletia excelsa) and leucaena (Leucaena leucocephala) leaf meal fed to tambaqui (Colossoma macropomum) and on fish growth. Five treatments were used: UPI denoted the negative control diet with unfermented plant ingredients; UPIP was positive control diet with unfermented plant ingredient supplemented with 3 g/kg phosphorus (NaH 2 PO 4 ); FPI indicated diet with fermented plant ingredients; FPIPT1 denoted diet with fermented plant ingredients supplemented with phytase 4000 U/kg; and FPIPT2 was diet with plant ingredients fermented together with phytase 4000 U/kg. Chromic oxide was added to the diets as inert marker for digestibility study. The diets were fed to juveniles of tambaqui (70.0 ± 6.3 g) to satiation twice daily in recirculation systems for 56 days. Results indicated high and better digestibility of protein and lipid by fish fed diet with phytase, which resulted in a corresponding better fish growth. Growth rate of the fish was high and increased from initial weight of 70 g to 132 g almost doubling the initial weight. No significant differences were found in weight gain, specific growth rate, and feed conversion ration, but performance was better in fish fed diets with phytase. The fish grew well in low dissolved oxygen levels of 4.23-4.34 mg/l, pH of 5.80-5.91 and high ammonia concentration of 1.76-2.17 mg/l without compromising growth and physiological functions.