Cry1Aa is an insecticidal protein produced by Bacilllus thuringiensis. To elucidate the mechanisms of cell/individual death and healing in the midgut epithelium, Bombyx mori larva were given different concentrations of Cry1Aa, and sections from the midgut were examined by terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining. In the lethal condition, most midgut columnar cells were observed to swell and burst without being stained by TUNEL 4 h after intoxication, and the epithelial layer collapsed by 24 h. In the sublethal to nonlethal conditions, midgut columnar cells did not swell, but their nuclei were intensely stained by TUNEL at 4–24 h after intoxication. These apoptotic cells dropped out of the epithelial layer, and the epithelial layer was finally renewed by newly mature columnar cells within 72 h after intoxication. These results suggest that the reaction, which occurs in the midgut of intoxicated insects, is determined by the interplay between passive toxic action and the active host healing response, in a toxin concentration-dependent manner. To investigate the signal pathways regulating apoptosis and the proliferation of the intoxicated epithelial layer, the expression of several genes was further examined by quantitative real-time PCR. Bm-Apaf1 was the most upregulated gene postintoxication.