Abstract: A strain of Beauveria bassiana Bb00,isolated from the soil,was used as the original isolate R0.Isolates R1,R2,R3 and R4 were obtained by repeatedly inoculating the A.germari larvae with R0,and isolates M1,M2,M3 and M4 were obtained by repeatedly growing R0 on Czapek medium.Isolates R0,R2,R4 and M2,M4 were tested for their infectivity to the A.germari larvae.The results showed that the pathogenicity of the successive isolates M2,M4 to the A.germari larvae reduced,while that of the isolates R0,R2 and R4 increased.The phenoloxidase activity (POA) in hemolymph of A.germari larvae infected by B.bassiana increased within the early days of infection because of the immunoreaction,then decreaed because of the adaptability of B.bassiana.The POA in hemolymph of A.germari larvae infected by successive isolates obtained from Czapek medium was higher than that recovered from A.germari larvae.There were some correlations between the time producing maximal phenoloxidase activity and the LT50 value of different isolates,which reflects different infective speed.At the same time,a close relationship between the maximal activity and the LC50 value was also found.All this indicated that the phenoloxidase activity in the hemolymph of A.germari larvae can not only weigh the immunity ability of the host in the process of B.bassiana infection,but also act as an important reference index of the pathogenicity of B.bassiana.