Insecticides derived from the common soil bacterium Bacillus thuringiensis (Bt) are becoming increasingly important for pest management. Insecticidal crystal proteins (also called S-endotoxins) from Bt are extremely toxic to certain pests, yet cause little or no harm to humans, most beneficial insects, and other nontarget organisms (19, 40). After proteolytic activation in the insect midgut, Bt toxins bind to the brush border membrane of the midgut epithelium and create pores that cause cells to swell and lyse (55). Technical innovations, including expression of Bt toxin genes in transgenic crop plants and transgenic bacteria, should increase the usefulness of Bt (6, 12, 35, 46, 91, 94, 109, 116). At the same time, mounting concerns about environmental hazards and widespread resistance in pest populations are reducing the value of conventional synthetic insecticides.