A certain plasticity of gonadal sex differentiation was reported in the 1930s following exogenous steroid treatments in fish, but demonstration that some environmental factors (temperature, pH, density and social interactions) can influence the sex ratio in gonochoristic species, is relatively recent. In fish, as in reptiles and amphibians displaying environmental sex determination the main environmental factor influencing sex seems to be temperature (TSD = Temperature Sex Determination). In most of the thermosensitive species (some Atherinids, Poecilids, Cichlids, the tilapias, the goldfish, a Siluriform, a flatfish,...) the male to female ratio increases with temperature, and/or ovarian differentiation is induced by low temperatures. Conversely, in some rare species (Dicentrarchus labrax, Ictalurus punctatus), high temperatures may produce female-biased sex ratios and/or low temperatures promote male-biased sex ratios. In the case of the hirame Paralichthys olivaceus, both high and low temperatures induce monosex inale populations while intermediate temperatures yield a 1 :I sex ratio (U-shape curve). Fish show particularities in their TSD patterns since monosex populations are generally not produced at extreme temperatures, suggesting the existence of strong temperature/genotype interactions. In reptiles, amphibians as well as fish displaying TSD, temperature treatments must be applied at a critical sensitive period which is relatively similar to the hormone-sensitive period. In gonochoristic fish, steroid hormones with estrogens in females and II-oxygenated androgens in males, are probably key physiological steps in the regulation of the gonadal sex differentiation. Cytochrome P450-aromatase, an enzyme which catalyses the conversion of androgens to estrogens, seems to be a critical enzyme for ovarian differentiation. The molecular mechanisms of thermosensitivity have been addressed in only two species. In both the tilapia, Oreochromis niloticus and the hirame, a