It was recently discovered for gypsy moths (and suggested to possibly be the case for other insects, as well) that the B. thuringiensis toxin does not kill the larvae by itself as previously thought. Rather, bacteria that are part of the insect’s gut microbial community are required for toxicity to the insect. Elimination of the insect’s gut bacteria by oral administration of antibiotics abolished B. thuringiensis insecticidal activity, and reintroduction of an Enterobacter sp. that is normally part of the insect’s gut microbial community restored this activity. The data indicate that the B. thuringiensis toxin enables the enteric bacteria to reach the insect hemocoel by permeabilizing the gut epithelium. In this way, the insect is killed much more rapidly than might otherwise be expected. The discovery that B. thuringiensis insecticidal activity depends on insect enteric bacteria should not have any significant effect on the efficacy or use of B. thuringiensis-based insecticides. However, this information may be important in the design and execution of some laboratory experiments intended to better understand the functioning of B. thuringiensis insecticidal strains and to facilitate the development of improved biological insecticides.