this experiment was conducted the norte fluminense state university darcy ribeiro in the aquaculture sector, aiming to evaluate the supplementation of phytase for juvenile tambaqui (colossoma macropomum). it was used a total of 170 fingerlings with average initial weight of 3.12±0.05 g and initial total length with the average of 5.36±0.06 cm. the fingerlings were randomly distributed in 20 experimental tanks with an useful volume of 40 l, being the water supplied from a closed circulation system, equipped with a biological filter. the treatments consisted of five isoproteic and isocaloric diets (28% protein and 3100 kcal ed/kg diet), supplemented with four levels of exogenous phytase (700, 1400, 2100 and 2800 uaf/kg diet) with one more without supplementation. the addition of phytase to the diets was done during the mixing of the ingredients for the manufacture of pelleted diets. it was verified the effect of phytase supplementation on the measured variables, being the smallest estimated dose equal to 1540.62 ufa kg-1 of diet for feed intake variable and the higher estimated dose equal to 2303.83 ufa kg-1 of diet for variable rate of specific development. the doses estimated in this study are much higher than those found in studies that mention the effect of supplementation on the fish, although the dose of supplementation, as well as age and other factors may be grounds, or not, for checking the action of phytase on the development of fish. at the end of the experiment, it was concluded that phytase influenced the development of tambaqui from the estimated dose, 1540.62 ufa kg-1, of diet.