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 the first 12 h. During the remainder of the 3-d observation period, larvae on the Cry 1Ac diet showed some recovery of feeding behavior but after 3 d had a similar mean weight to that of starved larvae. After 3 d on the Cry 1Ac diet or starving for 3 d, larvae were allowed to feed on the control diet for a further 7 d. Midgut cells of larvae of both treatments recovered to normal size and shape, but larvae initially given Cry 1Ac diets made a significantly greater weight gain than larvae that initially starved.