Differentiated cells in the insect midgut depend on stem cells for renewal. We have immunologically identified Integrin β₁, a promotor of cell–cell adhesion that also induces signals mediating proliferation, differentiation, and apoptosis on the surfaces of cultured Heliothis virescens midgut cells; clusters of immunostained integrin β₁–like material, indicative of activated integrin, were detected on aggregating midgut columnar cells. Growth factor–like peptides (midgut differentiation factors 1 and 2 [MDF1 and MDF2]), isolated from conditioned medium containing Manduca sexta midgut cells, may be representative of endogenous midgut signaling molecules. Exposing the cultured midgut cells to Bacillus thuringiensis (Bt) toxin caused large numbers of mature differentiated cells to die, but the massive cell death simultaneously induced a 150–200% increase in the numbers of midgut stem and differentiating cells. However, after the toxin was washed out, the proportions of cell types returned to near-control levels within 2 d, indicating endogenous control of cell-population dynamics. MDF1 was detected immunologically in larger numbers of Bt-treated columnar cells than controls, confirming its role in inducing the differentiation of rapidly produced stem cells. However, other insect midgut factors regulating increased proliferation, differentiation, as well as inhibition of proliferation and adjustment of the ratio of cell types, remain to be discovered.