Virulence is one of the major biological properties of pathogen involved in causation of disease. Among the selected bivoltine breeds whose LC for 30, 50, 60, 70 and 80 dosages by inoculation of BmNPV PIBs were determined, 61N, CSR19 and S1 require higher dosage of inoculum to cause LC 30, 50, 60, 70 and 80% mortality than other breeds CSR18 and CSR4. The LC 30, 50, 60, 70 and 80 dosage for 61N, CSR19 and S1 were S1 104.711, 105.360 and 104.579; 106.538, 106.539 and 106.400; 107.265, 107.417and 107.276; 108.366, 108.059 and 108.222; and 109.491, 109.000 and 109.343 BmNPV PIB/ml respectively. The results of  LC for 30, 50, 60, 70 and 80 by inoculation of BmNPV PIB and low temperature induction, 61N, CSR19 and S1 were observed to require higher dosage of BmNPV PIB inoculum than the breeds CSR18 and CSR4. The LC 30, 50, 60, 70 and 80 dosage required by 61N, CSR19 and S1 required for 30, 50, 60, 70 and 80% mortality were 100.169, 100.338and 100.058; 101.593,  102.212and  101.471; 102.441, 103.439 and 102.206; 103.358, 104.763 and 103.000 and 104.441, 106.333 and 103.941 BmNPV PIB/ ml respectively. The LC 30, 50, 60, 70 and 80 dosage for the breed CSR18 and CSR4 were much lower and it was for 30, 50, 60, 70 and 80% mortality were  100.438 and 100.424;  100.717 and 100.928 ; 101.990 and  101.576, 102.123 and 102.277 and 103.219 and 103.108 , respectively. The results indicated 61N, CSR19 and S1 were comparatively tolerant for their resistance to BmNPV based on low (LC30), medium (LC50) and high (LC80) dose. The overall results indicated that 61N and CSR18 were comparatively higher tolerant and were selected for enhancing the resistance against BmNPV.