Abstract: After infecting with nuclear polyhedrosis virus (AsNPV), granulosis virus (AsGV) or cytoplasmicpolyhedrosis virus (AsCPV) of Agrotis segetum, the respiratory rate (RR) within 120hr to 240hr in cutwormlarvae was lower than that of the control in spite of a similar trend was observed in both two cases.The freshwight of larvae infected with AsGV decreased gradually with the prolongation of its development and showed apositive relation to RR change.The changes of RR in larvae infected with either AsNPV or AsCPV were negative relation to the changes of their fresh weight.The peaks of the RR curve in larvae infected with the viruseswere coincided with the logarithmic phases of the virus replication.Significance of difference test in the infected and uninfected cases showed that the change of larvae fresh weight was more obvious than that of RR, andcould be used for judging the infection of larvae but could not be as the index of the infected situation.