BmNPV (Bombyx mori nuclear polyhedrosis virus) causes nuclear polyhedrosis in silkworms. This paper reports on the relative susceptibility of silkworm pure breeds and their hybrids reared under BmNPV stress condition. Infection during fourth and fifth instar silkworm Bombyx mori L., with nuclear polyhedrosis virus caused reduction in mature larval weight and increased in ET50 for mortality. The mature larval weight of fourth and fifth instar silkworm revealed significant results. However, maximum (26.28 & 28.35g/10) and (25.75 & 26.33 g/10) was encountered for 10-1 and 10-3 fed batches of PMxCSR16 & PMxCSR2 were noticed. Among pure breeds, PM has recorded significantly lesser larval weight (16.68 & 17.10g/10) and (18.39 & 19.58g/10) compared to other pure breeds. Whereas, CSR2xCSR4 (10-1) in fourth instar and CSR2xCSR16 in both the instar at (10-1 and 10-3) both the dilutions did not recorded mature larval weight due to its high sensitivity to BmNPV. The time taken for 50 percent mortality of test population in bivoltine pure breed recorded maximum (9.67 days) and minimum (7.00) days for CSR4 & CSR16 and CSR4 (9.67 days) & CSR2 (7.67 days) of both the viral dilutions respectively. In fifth instar irrespective of genotypes and their hybrids 9.67 and 11.0 days ET50 value were recorded. The silkworm hybrids were shown comparatively higher ET50 values compared to their genotypes.