One of the main problems to be solved in the commercial production of Nile tilapia is the use of large volumes of steroids during the sex reversion process. The precise evaluation of the time after hatching to start the steroid treatment could help reduce its use in the short-term. The main objective of this work was to evaluate the sex proportion and the growth obtained in a mixed population of Nile tilapia fed fluoxymesterone at different days (3, 6, 9, 12, and 15) after hatching. A one-factor experiment was designed, with the day, after hatching, that the fluoxymesterone (5 mg) began to be supplied as factor (3, 6, 9, 12 and 15). Initial stocking density was 0.4 larvae L -1 . All groups were evaluated by triplicate. Steroid treatment finished at day 25 after larvae stocking. Squash technique was used to determine sex proportion at the end of the experiment (45 days). Growth evaluation was performed based on wet weight and total length obtained from each biometry. The results showed the highest percentage of males (92%) in the fish fed fluoxymesterone from day three after hatching. No anabolic effect on growth was observed in any steroidtreated fish. A negative effect on growth was observed after withdrawal of the steroid in the fish that were fed fluoxymesterone for the longest time. These results show that it is necessary to supply the fluoxymesterone prior to the onset of sex differentiation (5-6 days after hatching) to achieve a permanent reversion of the sex differentiation process.