Ultrastructural changes of tissues caused by Bacillus thuringiensis var. kurstaki $delta$-endotoxin intoxication of Hyphantria cunea were observed by transmission electron microscopy. Bt $delta$-endotoxin crystals induced the disruption of microvilli, vacuolation of cytoplasm, changes in the cisternae of the endoplasmic reticulum, disappearance of basal striations, loss of ribosomes, and changes in the configurations of mitochondria in the columnar cell of midgut. The fat body cells also showed spherical endoplasmic reticulum and distorted mitochondria, and then the cells were destroyed.