European sea bass (Dicentrarchus labrax L.) is a species with group-synchronous ovarian development. A mechanism is required which enables maturation to occur in the first clutches of oocytes without inducing maturation in subsequent clutches. The present study examined the individual plasma variations of testosterone (T), 17,20b-dihydroxy-4-pregnen-3-one (17,20bP) and 17,20b,21-trihydroxy-4-pregnen-3-one (20bS) in both sexes, estradiol (E2) and vitellogenin (VTG) in females (n=15) and 11-ketotestosterone (11KT) in males (n=21), in an effort to elucidate the hormonal control of the reproductive cycle in this species. A sample of oocytes was obtained at every sampling from each female and the stage of development of the most advanced clutch of oocytes was determined and related to the individual hormone plasma levels. Total expressible milt was collected from males at each sampling during the spawning period and variations in the sperm production were related to hormone plasma levels. Successive elevations of plasma T and E2 levels were observed prior to peaks of progestagens, which resulted from the shift in gonadal steroidogenesis and coincided with the maturation-ovulation of the different clutches of oocytes or with increases in the sperm production. Following each progestagens wave, a new shift in gonadal steroidogenesis, resulting in a new elevation in plasma T and E2, was observed. This hormonal pattern was repeated several times depending on the number of ovulations per female. Results from the present study suggest a mechanism, based on shifts in gonadal steroidogenesis, which may be responsible for regulation of group-synchronous ovarian development, ovulation and spermiation in this species. In addition, evidence is presented which supports a role for both 17,20bP and 20bS as the maturation-inducing steroids (MIS) in male and female European sea bass.