To evaluate the effect of dietary lysozyme on growth and anti-infection ability of grass carp,Ctenopharyngodon idellus,fish with initial body weight 6.93±0.27 g were randomly divided into six groups and fed diets with 0 (control),100,200,300,400,500 mg/kg dietary lysozyme respectively for 60 days.The results showed that the growth performance and anti-infection ability of grass carp were significantly affected by dietary lysozyme.In the first 45 days of the feeding trial,the specific growth rate (SGR) in all lysozyme-treated groups was significantly lower than that in the control group (P<0.05).At the end of the feeding trial,the SGR in the 100-300 mg/kg groups was not significantly different from the control,while the SGR in the 400 and 500 mg/kg groups was significantly lower than that in the control (P<0.05).Feed conversion ratio decreased first,then increased with the increment of the dietary lysozyme,with the lowest in the group of 200 mg/kg.After the tested fish was challenged with live Aeromonas hydrophila by intraperitoneal injection,mortality rate of the 200-500 mg/kg groups was significantly lower than that of the control and 100 mg/kg group.This study further indicated that activities of endogenous digestive and immune enzymes were enhanced by the dietary lysozyme to some extent.Lysozyme was also found to have strong inhibitory activity against common Gram negative bacteria in aquaculture by antimicrobial tests.In summary,the optimal level of dietary lysozyme was 200-300 mg/kg according to the growth performance and the anti-infection ability.