A feeding experiment was conducted to determine the dietary requirement of threonine of juvenile Japanese seabass(initial average weight 8.00?0.20 g).Six isonitrogenous and isoenergetic semipurified diets were formulated to contain 41.83% crude protein,19.68% kJ/g gross energy and six levels of threonine 0.75%,1.03%,1.36%,1.67%,2.04% and 2.31%(dry matter basis)at about 0.35% increment(diets 1-6).Each diet was randomly assigned to triplicate groups of 30 fish and fed to apparent satiation by hand twice daily(06:00 and 17:30)for 8 weeks in floating sea cages.The water temperature fluctuated from 26 to 32 ℃,salinity from 22‰ to 28‰ and dissolved oxygen was approximately 7 mg/L during the experimental period.No significant differences in survival were found among dietary treatments.Weight gain rate(WGR)significantly increased with increasing dietary threonine from 150.3 to 256.9%(P<0.05),and thereafter showed a declining tendency.Nitrogen retention(NR)significantly increased with increasing dietary threonine from 13.18% to 26.36%(P<0.05), same as above,declined.No significant differences in body composition were found among dietary treatment.On the basis of either WGR or NR,the optimum dietary threonine requirements of juvenile Japanese seabass were estimated to be 1.77% of diet(4.21% of dietary protein)and 1.88% of diet(4.47% of dietary protein),respectively,using second-order polynomial regression analysis.