Abstract: Under laboratory conditions , the effect of crowding stress on the non2specific immune system and the resistance to fish pathogenic bacterial infection of grass carp ( Ctenopharyngodon idellus) was investigated. We made an inter2group difference by feeding grass carp ( C. idellus) in different densities and time to simulate the crowding stress encountered by aquaculture fish. After stress of 7 , 14 , 30 , 60 days , fish were sampled to examine their immune indexes. The results indicated that the lysozyme activity in serum of high density group (HG) increased after stress and reached a maximum on 14th day , then decreased over time. Plasma bactericidal activity reduced sharply when density increased. Leucocrit value did not show a significant difference among all groups , but phagocytic activity of macrophages from head kidney after crowding stress of 30 and 60 days showed a difference between HG and control group (CG) ( P < 0. 05) . After 60 days stress , fish in each group were challenged with Aeromonas hydrophila by intraperitoneal injection. Mortality rate of HG was 75. 0 % , middle density group (MG) 63. 6 % and control group only 25. 0 %. At the same sampling time , spleen weight indexes of both HG and MG were much lower than CG ( P < 0. 01) . Additionally , cortisol concentration in serum of all experimental groups came to their maximum values at the beginning of the stress , then dropped and recovered to normal level at the end of experiment .