Larvae of the black cutworm, Agrotis ipsilon Hufnagel, were more susceptible to infection by Agrotis ipsilon multiple nucleopolyhedrovirus (AgipMNPV) after feeding on Herculex ^® I, a transgenic corn hybrid expressing Cry1Fa2 compared to larvae fed on isoline corn. Experiments were conducted to investigate the physiological basis for increased susceptibility to virus infection following exposure to Herculex ^® I. Midgut pH, gut protease activity and peritrophic matrix structure are important factors for both Bt toxic action and baculovirus infection. No significant treatment differences were found in the pH of anterior midgut, central midgut or posterior midgut in larvae fed Herculex ^® I or isoline diets. Analysis of soluble and membrane–associated gut proteinase activities from larvae fed Herculex ^® I or isoline diets indicated that membrane–associated aminopeptidase activity and soluble chymotrypsin–like proteinase activity were significantly lower in Herculex ^® I –fed larvae compared to isoline–fed larvae. The number and relative molecular masses of soluble chymotrypsin–like proteinases did not differ. An experiment to determine whether chymotrypsin degradation of baculovirus resulted in reduced infection of larvae fed on isoline diet showed that baculoviruses are not susceptible to degradation by chymotrypsin. Analysis of the peritrophic matrices of Herculex ^® I –fed larvae and isoline-fed larvae by scanning electron microscopy indicated that Herculex ^® I did not result in damage to the peritrophic matrix that could facilitate subsequent baculovirus infection. Further analyses are required to determine whether Bt toxin–induced epithelial cell sloughing enhances subsequent virus infection.