In vertebrates, steroids play a major role in the complete sex differentiation; however, in teleost fish, no definitive physiological evidence supports the hypothesis that steroids are the natural inducers of gonadal sex determination. We investigated the role of androgens in 2 additionnal teleost models, Oncorphynchus mykiss and Oreochromis niloticus. The former presents a clear female homogamety, and the latter had a more complex sex derterminism since sex differentiation can be influenced by temperature. First we identified through in vitro studies, sex-specific androgens which could be synthetised by the gonads during male differentiation. In both species, 11-oxygenated androgens present a high specificity. The administration at low dosages via the alimentation of this steroid to genetically monosex female populations results in functional sex reversal in both species: male monosex populations are produced with similar dosages than artificial steroids as 17alphaMT. However, specific inhibitor of 11beta-hydroxylase activity (the enzyme involved in 11betaOHdelta4 synthesis) did not modify the sex-ratio; effects of this inhibitor on 11betaOHdelta4 synthesis has been analysed by RIA in the rainbow trout. Moreover, in vivo treatment of male monosex population with an androgen receptor antagonist, Flutamide, failed to induce any feminisation. Gonad contents have been measured by RIA in genetically male or female monosex populations from both species from undifferentiated stages to first gametogenesis. At last, long term whole gonad culture of genetically all-female gonads from histologically undifferentiated tilapia fry have been realised in presence or in absence of 11betaOHdelta4. Oocytes are observed in both groups of whole gonads after a 23 days period of in vitro culture. The role of 11beta-androgens on sex differentiation is discussed.