Abstract: To study the effect of dietary CHO level on immunity in the southern catfish, the serum lysozyme activity, antibacterial activity and total serum protein concentration were measured in the fish fed with the control, middle level and high level carbohy drate (CHO) diets containing 0 %, 15 % and 30 % gelatinized corn starch, respectively The southern catfish,Silurus meridionalis Chen (207±015) g were acclimated with the control diet for 15d at 2715±012℃1The feeding growth experiment lasted for0, 2, 4, 8, 12 and 16 weeks in the recirculated water system1The lysozyme activity in the fish at middle CHO level was signifi2cantly lower than that in the control group only at the 4th and the 12thweek (p < 0105),and lysozyme activity in the fish at highCHO level was significantly lower than that in the control group from the 2ndweek to the 12thweek (p < 0105),while no signifi2cant difference was found between the two groups at the 16thweek. Antibacterial activity in the fish at either middle or high CHOlevel was significantly lower than the control from the 4thto 16 thweek (p < 0105),but there was no significant difference betweenthe former two groups. The total serum protein concentration in the fish at high CHO level and the middle CHO level were signifi2cantly higher than the control only at 16thweek (p < 0105),and there was no significant difference among the three groups at oth2er sampling time. Although negative influence of dietary CHO of 15 % on immunity was observed in the southern catfish, growthperformance was nor affected1And the negative influence increased with increasing CHO level. It can be concluded that the CHO is an important factor that restrains the immunity in the southern catfish. Accelerating synthesis of lysozyme protein should be one of the adaptive mechanisms in the fish under a long term of dietary CHO stress, which resulted in the increased serum protein. The results indicated that an optimum carbohydrate diet should be evaluated by examining its influence on both growth and immu2nity in this fish.