The paper was to investigate the control of sexual differentiation by oral administration of varying doses of 17β2estradiol ( E2) in juvenile grey mullet , Mugil cephalus with sexual undifferentiation. The results indicated that the grey mullets in the high dosage group were induced to be all2female , and 90 % of the juvenile grey mullets in the low dosage group differentiated into female. The control group differentiated into all2males . Sexual differentiation into females occured significantly earlier than in males. The residue level and residue time of 17β2 estradiol were low and short in the muscle and gut of grey mullets.