Two breeds ofBombyx mori, one resistant (Nistari) to BmNPV(Bombyx morinuclear polyhedrosis virus) and the other sensitive (Hosa Mysore), were inoculated with the virus, and changes in the levels of alkalinity and the level of alkaline protease activity were estimated separately at different time intervals. The midgut epithelial cells and fat bodies were also examined at transmission electron microscope at different intervals after inoculation with BmNPV to understand the mode of entry and multiplication of the virus. The alkaline protease activity in the midgut of the tolerant breed showed a sharp increase up to three days after inoculation, which was significantly higher than the increase in the fore‐gut and hindgut and than that of the sensitive breed. The ultra‐structural studies showed that the invasion and multiplication of the virus take place only in the sensitive breed. The study indicated that the tolerance to BmNPV infection is positively correlated with alkaline protease activity, and that the enzyme is more active in the midgut of the tolerant breed.