This study was directed to identify the major steroid produced by follicles of the Siberian sturgeon during oocyte maturation. Among several steroids produced in vitro the 17,20β,21-trihydroxy-4-pregnen-3-one (20βS) was the most prominent both under basal and LH-stimulated conditions; while the 17,20βP was not systematically detected in the plasma of GnRHa-stimulated females. These results suggest that 20βS could be the maturing inducing steroid (MIS) for the species.