An 8-week feeding experiment was performed to evaluate the effect of dietary genistein on growth performance,body composition,and digestive enzymes activity of juvenile Nile tilapia(Oreochromis niloticus).Four isonitrogenous and isoenergetic diets were formulated containing four graded supplements of genistein:0,30,300,and 3 000 μg/g.Each diet was randomly assigned in triplicate to tanks stocked with 15 juvenile tilapia(10.47±1.24 g).The results show that 30 and 300 μg/gdietary genistein had no significant effect on growth performance of Nile tilapia,but the higher level of genistein(3 000 μg/g)significantly depressed the final body weight and specific growth rate.There was no significant difference in survival rate,feed intake,feed efficiency ratio or whole body composition among all dietary treatments.An assay of digestive enzymes showed that the diet containing 3 000 μg/ggenistein decreased stomach and hepatopancreas protease activity,and amylase activity in the liver and intestine,while a dietary level of300 μg/g genistein depressed stomach protease and intestine amylase activities.However,no significant difference in stomach amylase activity was found among dietary treatments.Overall,the results of the present study indicate that a high level of dietary genistein(3 000 μg/g,or above) would significantly reduce the growth of Nile tilapia,partly because of its inhibitory effect on the activity of major digestive enzymes.Accordingly,the detrimental effects of genistein,as found in soybean products,should not be ignored when applied as an alternative ingredient source in aquaculture. An 8-week feeding experiment was performed to evaluate the effect of dietary genistein on growth performance, body composition, and digestive enzymes activity of juvenile Nile tilapia (Oreochromis niloticus). Four isonitrogenous and isoenergetic diets were formulated containing four graded supplements of genistein: 0 , 30,300, and 3 000 μg / g. Each diet was randomly assigned in triplicate to tanks stocked with 15 juvenile tilapia (10.47 ± 1.24 g). The results show that 30 and 300 μg / gdietary genistein had no significant effect on growth performance of Nile tilapia, but the higher level of genistein (3 000 μg / g) significantly depressed the final body weight and specific growth rate. There was no significant difference in survival rate, feed intake, feed efficiency ratio or whole body composition among all dietary treatments . An assay of digestive enzymes showed that the diet containing 3 000 μg / ggenistein decreased stomach and hepatopancreas protease activity, and amylase activity in the liver and intes tine, while a dietary level of 300 μg / g genistein depressed stomach protease and intestine amylase activities. Even, no significant difference in stomach amylase activity was found among dietary treatments. Overall, the results of the present study indicate that a high level of dietary genistein (3 000 μg / g, or above) would significantly reduce the growth of Nile tilapia, partly because of its inhibitory effect on the activity of major digestive enzymes. Accreditedly, the detrimental effects of genistein, as found in soybean products, should not be ignored when applied as an alternative ingredient source in aquaculture.