Follicle-stimulating hormone (FSH) is a glycoprotein hormone that plays a key role in the regulation of gonadal functions in vertebrates. The present study reports the monitoring of pituitary and plasma Fsh levels during sex differentiation and oogenesis in European sea bass (Dicentrarchus labrax) using a homologous immunoassay and an in vitro bioassay. Both assays were used complementarily for the first time in a fish species. High levels of Fsh bioactivity in plasma were found during the initial phases of sexual differentiation. Plasma and pituitary Fsh (quantity and bioactivity) levels and biological to immunological (B:I) ratios were higher in females than in males, suggesting sexual dimorphism in the synthesis and potency of Fsh. In females, the B:I ratios in adult were lower than during sex differentiation indicating that Fsh would be less biopotent in the adult stage. Plasma Fsh bioactivity levels increased during vitellogenesis, suggesting that Fsh would be involved in the regulation of the midphases of oogenesis, whereas luteinizing hormone would be responsible for the final events.