Abstract: A 12-week feeding trial was conducted to evaluate the effects of dietary daidzein on growth performance,activities of digestive enzymes,anti-oxidative ability and intestinal morphology in juvenile turbot(Scophthalmus maximus L.)Mean initial body weight(4.63±0.01)g.Five isonitrogenous and isolipidic diets(crude protein,50%;crude lipid,10.5%)were formulated to contain 0,5,10,20,and 100 mg/kg daidzein,respectively.The feeding trial was conducted in an in-door flowing seawater system.Triplicate groups of 30 fishes were hand-fed to apparent satiation twice daily.After the feeding trial,growth performances,physiological and biochemical parameters were analyzed.The results showed that the growth performances and feed utilization of fish,including final weight(21.24-24.42 g),specific growth rate(1.81-1.98 %/d),feed efficiency(1.01-1.11)and feed intake(1.43-1.51 %/d),were not significantly affected by dietary daidzein supplementation(P>0.05).Similar results were also found in condition factor(1.83%-1.99%),hepatosomatic index(0.72%-0.97%)and viscerosomatic index(4.19%-4.68%)(P>0.05).However,the addition of 5-100 mg/kg daidzein in diet significantly decreased the protein(15.41%-15.59%)and lipid(3.19%-3.93%)contents of fish whole body(P<0.05)without effects on the moisture(77.41%-79.70%)and ash(3.46%-3.81%)contents(P>0.05).The trypsin(35.26-40.66 U/g prot)activity in turbot was significantly(P<0.05)improved by 10-100 mg/kg dietary daidzein whereas the activities of pepsin(31.75-49.56 U/mg prot),intestinal caseinolytic(10.00-14.79 U/mg prot),intestinal amylase(0.05-0.17 U/mg prot)and gastric amylase(0.10-0.25 U/mg prot)showed no significant difference among all treatments(P>0.05).Compared to the control,addition of 5-20 mg/kg daidzein in diet significantly increased the activities of serum superoxide dismutase(SOD)(51.05-53.36 U/mL)and glutathion peroxidase(GSH-Px)(551.40 U/mL),and decreased the content of serum malondialdehyde(MDA)(10.67-11.17 nmol/mL)in fish(P<0.05).No lesion or damage was found on the distal intestine structures of fish from all treatments,however,the height of fish distal intestine fold(391.26-401.48 μm)was significantly increased(P<0.05)by 10-20 mg/kg dietary daidzein.These results suggested that dietary daidzein(5-100 mg/kg)did not affect growth performance of juvenile turbot,while moderate levels of daidzein(10-20 mg/kg)could increase the activities of digestive enzymes,and improve the anti-oxidative ability and intestinal morphology of turbot.The present study provided important information regarding the effects of daidzein on turbot as well as the use of soybean meal in aqua feed.