Abstract Effects of ingesting Cry 1Ac, a δ-endotoxin from Bacillus thuringiensis, on the larval midgut of lightbrown apple moth, Epiphyas postvittana (Walker), were studied using light and electron microscopy and related to feeding behavior and growth. After larvae had ingested Cry 1Ac, epithelial midgut cells displayed fine protrusions from microvilli and loss of microvilli, expulsion of columnar cells into the midgut lumen, and, finally, rupture and disintegration of these cells. All of these effects were observed within 1 h of larvae being placed on the Cry 1Ac diet. As larvae continued to feed on this diet for 3 d, the midgut underwent disruption and recovery cycles. Midgut changes, feeding behavior, and growth of these larvae were compared with those of larvae on control diets and larvae starved for 3 d. Starvation induced a decrease in the size of midgut cells but no cell lysis, rupture, or disintegration. Larvae on the Cry 1Ac diet produced more frass than starved larvae but ate very little during ...