The genetic and environmental factors underlying sex determination and differentiation in teleost fish are still a matter of active debate. Nevertheless, the current hypothesis is that ovarian aromatase (cyp19a) expression plays a key role in orienting the embryonic gonad towards the female phenotype. On another hand, it is now well known that the brain of teleosts has a very high aromatase activity and that brain aromatase (cyp19b) expression occurs early during development. However, the potential role of aromatase in brain sexual differentiation is totally unknown. In this study, we monitor, for the first time in teleosts, the expression of the main steroidogenic enzymes in genetic all-male and all-female populations. Brains were dissected from male and female larvae around the period of sexual differentiation between 37 and 54 days post-fertilization at 10°C. At each of these time points, samples were pooled (n=30 to 50 depending on the stage) and mRNA were extracted for real-time RT-PCR gene expression analysis of cyp19b (cytochrome P450 aromatase B), cyp11a1 (cholesterol side chain cleavage), hsd3b1 (3β-hydroxysteroiddehydrogenase), cyp17a1 (17-hydroxylase) and sf1 (nr5a1b, steroidogenic factor 1). Very interestingly, a clear sexual dimorphism was observed between males and females which, in general, showed expression levels lower than in males. This was particularly obvious in the case of cyp19b and cyp11a1, but also for cyp17a1, at least at the youngest stages. Of particular interest is the fact that measurements performed on the gonads of the same animals showed opposite results, i.e., higher gene expression of most steroidogenic enzymes in females than males. Strikingly, brain aromatase (cyp19b) expression is much higher in the brain of males than in the brain of females, whereas expression of gonadal aromatase (cyp19a) is much higher in the differentiating female gonad. These results demonstrate that, at the time of gonadal differentiation, the brain is already sexually differentiated with at least some genes encoding steroidogenic enzymes exhibiting a clear sexual dimorphism. This early brain molecular sex differentiation may actually be more pronounced as in addition to these results, we also identified using a global microarray approach some other genes with a clear sexually dimorphic expression.