Bacillus thuringiensis is the most widely applied biological insecticide and is used to manage insects that affect forestry and agriculture and transmit human and animal pathogens. This ubiquitous spore-forming bacterium kills insect larvae largely through the action of insecticidal crystal proteins and is commonly deployed as a direct bacterial spray. Moreover, plants engineered with the cry genes encoding the B. thuringiensis crystal proteins are the most widely cultivated transgenic crops. For decades, the mechanism of insect killing has been assumed to be toxin-mediated lysis of the gut epithelial cells, which leads to starvation, or B. thuringiensis septicemia. Here, we report that B. thuringiensis does not kill larvae of the gypsy moth in the absence of indigenous midgut bacteria. Elimination of the gut microbial community by oral administration of antibiotics abolished B. thuringiensis insecticidal activity, and reestablishment of an Enterobacter sp. that normally resides in the midgut microbial community restored B. thuringiensis-mediated killing. Escherichia coli engineered to produce the B. thuringiensis insecticidal toxin killed gypsy moth larvae irrespective of the presence of other bacteria in the midgut. However, when the engineered E. coli was heat-killed and then fed to the larvae, the larvae did not die in the absence of the indigenous midgut bacteria. E. coli and the Enterobacter sp. achieved high populations in hemolymph, in contrast to B. thuringiensis, which appeared to die in hemolymph. Our results demonstrate that B. thuringiensis-induced mortality depends on enteric bacteria.