Abstract: Galla chinensis can prevent fish gill-rot disease, but the using dosage is not clear. In order to ascertain the recommended dosage of Galla chinensis, this study examined the inhibitory effect of different doses of Galla chinensis on pathogentic bacteria and the effect of dietary supplementation of different doses of Galla chinensis on the preventive effect of grass carp gill-rot disease in vivo and in vitro. The Galla chinensis decoctions were added to Shieh agar selection medium at doses of 40, 60, 80, 100 mg/kg, coated with Flavobacterium columnare, cultured for 72 h and counted to calculate the inhibition rate; a total of 240 healthy grass carp juveniles were selected and divided into 4 groups, the control group was fed with basal diet, and the 3 test groups were fed with the basal diet adding 1%, 2%, and 4% Galla chinensis respectively, and the suspension of Flavobacterium columnare was sprayed into the fish tank for 3 consecutive days to challenge. After 56 days, the activity of lysozyme in serum, intestine and hepatopancreas, and the serum contents of complement C3 and IgM were measured. The results showed that Galla chinensis at 80~100 mg/kg had obvious antibacterial effect, and Galla chinensis at 100 mg/kg could completely inhibit the bacteria; the 2% Galla chinensis group had significantly （P<0.05） higher hepatopancreas lysozyme activity compared with that of the control group. Therefore, the recommended dosage of Galla chinensis in grass carp feed is 2%.

Key words: Galla chinensis, grass carp, antibacterial effect, gill-rot disease