The profitability of tilapia culture is directly influenced by the efficient control of its reproduction through artificial means. Thus, many studies have focused on gonadal differentiation and sex determination. In Oreochromis niloticus, XX sex reversed males (produced by hormone treatment and analysis of the sex ratio of their progenies from single pair matings) can be used as a reliable tool to analyze both the efficiency of masculinizing treatments and sex determination. Twentytwo broodfish from two sets of sibling sex reversed males were tested through 35 single pair matings by microscopic sexing of all 4,546 fry. The 35 crosses yielded progenies composed of a majority of females, but all-female progenies were observed in only five reproductions. The other progenies yielded sex ratios significantly different from 1:1, ranging from 65 to 99% female with an average of 85%. Three unexpected males from these crosses were further tested; their progenies also included unexpected males. These results cannot be explained by a simple monofactorial sex determining system. Environmental factors may be involved in the sex determining mechanisms of O. niloticus.