The European sea bass (Dicentrarchus labrax L.) is a differentiated gonochoristic marine teleost of the family Moronidae (closely related to the hermaphrodites of the family Serranidae), where many juvenile males exhibit intratesticular oocytes, suggesting a certain sexual lability. Like most fish, the sea bass does not have recognizable heterochromosomes or sex-linked markers but there are clear parental effects on the sex ratios. The data available so far indicate that the proportion of females resulting from individual crossings may range from as little as 1 to about 70%. Sex differentiation proceeds in a caudo-cranial fashion and starts when fish reach 8-9 cm standard length (usually about 200 days post-hatching, dph, under typical rearing conditions), with females differentiating first. Both forms of aromatase have been cloned in this species and their temporal expression has been studied. Brain aromatase is detectable already in the larval stages but its involvement in sex differentiation is not yet clear. The ovarian form increases after 100 dph before ovarian differentiation, with high levels in females and basal levels in males. Thus, ovarian aromatase seems to be involved in female differentiation. On the other hand, androgen receptor (AR) gene expression levels show the opposite pattern, with higher levels in males than in females. It is not yet known whether androgens are necessary for testicular differentiation or rather they are the result of it. Of the several environmental factors tested, temperature is the only one that has been shown to be able to clearly influence sex ratios. Larval and juvenile sea bass reared in captivity at high temperature usually develop as males. Recent research suggests that the high incidence of males under aquaculture conditions is due to the high water temperature used, and that the effects of temperature would be mediated by an inhibition of aromatase mRNA expression and activity in genotypic females. However, other effects of temperature mediated through alterations in developmental rates cannot be discarded. This paper reviews the current knowledge on sex determination and differentiation in the sea bass and suggests some directions for future research.