Healthy red swamp crayfish,Procambarus clarkii were artificially infected by injected with Aeromonas hydrophila at the concentration of 1.5×104（GroupⅠ） and 1.5×106 cfu/mL（GroupⅡ）.Immunity factors including total haemocyte counts (THCs),phenoloxidase (PO) activity,superoxide dismutase (SOD),lysozyme activity and antibacterial activity in the haemolymph were examined at 1-96 h post infection at regular time.The results showed significant differences were observed in immune indicators of two infected groups during 8-48 h post infection.THCs and SOD activities of two treatment groups significantly decreased after 8-48 h compared to control,and THCs,SOD activities of groupⅠ decreased to the lowest level after 12，24 h,and returned to original level after 48，96 h respectively.However,THCs,SOD activities of groupⅡ decreased to the lowest level after 48 h,and recovered gradually after 96 h.Lysozyme activity，antibacterial activity and PO activity of groupⅠwere significantly higher than those of the control group after 12-48 h,and reached to the top level after 24，24 and 48 h,respectively,and finally returned to original level after 96 h.Significant decrease was observed in lysozyme activity，antibacterial activity and PO activity of groupⅡ after 12-48 h,and the lowest level was observed after 48 h,then gradually recovered after 96 h.