In this paper,larvae of crayfish Procambarus clarkii were exposed to different concentrations of M.aeruginosa (1.0×10⁶,5.0×10⁶,1.0×10⁷ and 2.0×10⁷/mL)to investigate their impacts on survival and the hepatopancreas ultrastructure of the crayfish.At the same time,adult crayfish were exposed to the same different concentrations of M.aeruginosa and the total hemocyte counts density(THCs),content of hemocyanin in the serum,superoxide dismutase(SOD),peroxidase(POD),phenoloxidase(PO) and Na⁺/K⁺-ATPase in the gill filament were detected.The result shows that:(1) M.aeruginosa can significantly reduce the survival rate of crayfish larvae.When algae cells reached 1.0×10⁷/mL,the survival rate was significantly lower than that of the control group on the 19^th day(P<0.05).When the experiment finished,the survival rate was 48%.(2) The hepatopancreas of crayfish larvae became darkened and hepatopancreas cells were observed to be damaged under the transmission electron microscopy.(3) When exposed to the highest concentration of M.aeruginosa,crayfish acted a stress reaction:THCs of the adult crayfish increased significantly and stayed at a high level after being exposed for 1 d.There were not significant differences for hemocyanin content at the beginning while significantly decreased on 5 d(P<0.05).The activity of T-SOD in serum was inhibited after 1 d,and was activated to a high level on 5 d.The activity of POD and PO showed fluctuation trend.The activity of Na+/K+-ATPase in gill dropped significantly after 1 d (P<0.01),then increased and stayed at a higher level finally.The result indicated that M.aeruginosa has negative impact on the survival of the juvenile crayfish and affects immunity of the adult crayfish which may cause loss of output of crayfish.