To verify the sex steroids which are involved in oocyte maturation of the blacktip grouper, , we incubated vitellogenic oocytes (0.41 and 0.50 mm in average diameter) in the presence of exogenous steroid precursor (-hydroxyprogesterone). Steroids were extracted, separated and identified by thin layer chromatography. The major metabolites produced were androstenedione, estradiol-, estrone and progestogens. Progestogen metabolites in the oocytes of 0.50 mm were more abundant than those of 0.41 mm. Also, we investigated the effects of human chorionic gonadotropin (HCG; 5, 50 and 500 ), -dihydroxy-4-pregnen-3-one () and -trihydroxy-4-pregnen-3-one (; 5, 50 and 500 , respectively) on oocyte maturation. In the oocytes of 0.41 mm, treatment with 50 IU HCG stimulated GVBD () compared with controls (, <0.05). In the oocytes of 0.50 mm, treatment of (50 and 500 ) stimulated GVBD ( and , respectively) compared with controls (, <0.05). Treatment with 500 IU HCG also stimulated GVBD () compared with controls (<0.05). Taken together, these results suggested that both HCG and were effective on in vitro oocyte maturation and may act as a maturation inducing hormone in blacktip grouper.