Sex in the Nile tilapia, Oreochromis niloticus, is governed by the interactions between a complex genetic sex determination system and the influence of temperature. High temperatures applied during a critical period of sex differentiation can induce masculinisation in some progenies originating from both domestic and wild stocks. Differences in thermo-sensitivity were observed between the wild populations and both paternal and maternal effects have been demonstrated, suggesting that it is a heritable trait. This has been confirmed by the development of divergent lines for thermo-sensitivity. Two thermo-sensitive divergent lines were selected with regard to their sex ratios showing high (=90% males) and low responses (only 54%) when treated at 36°C temperatures. At least one of the mechanisms by which temperature exerts its effects on sex differentiation is through a downregulation of aromatase Cyp19. Expression profiles were analysed in monosex XX populations treated at 27°C and 36°C temperatures showing that temperature up-regulates Sox9s, Amh, IGFs, Dax1 and down-regulates other genes such as aromatase Cyp19-1a. Variation in these gene expression levels was associated with the percentage of males obtained by temperature treatments. The divergent thermo-sensitive lines have been used to complement our study done on the monosex populations. Expression of Cyp19-1a was also analysed in these two divergent lines. (Texte intégral)