Susceptibility to Bacillus thuringiensis of mosquito and lepidopteran larvae is affected by feeding behaviour and nutritional value of the available food. Reduced mortality is attributed to feeding inhibition and dilution of the pathogen in the presence of nutritional and inert particles, which limit the amount of ingested toxin. These reasons are, however, not sufficient to explain the data presented here. Values of LC50 (the concentration that kills 50% of exposed population) of B. thuringiensis subsp. israelensis (Berliner) against Aedes aegypti (L.) larvae and of B. thuringiensis subsp. kenyae (Berliner) against Spodoptera littoralis (Boisduval) larvae were about 20-217 and 2.3-44-fold higher, respectively, in the presence of nutritional or biologically inert (non-nutritional) particles than without. The number of B. thuringiensis spores in carcasses of B. thuringiensis -killed A. aegypti and S. littoralis larvae were between 1.9 and 5.6-fold and between 8.5 and 12-fold higher, respectively, in the presence of particles than without. In all cases, non-nutritional particles better protected the exposed larvae than nutritious particles. We propose that another basic mechanism exists, that ingested particles protect midgut epithelial cells by covering their surface and thus preventing availability of the toxin to the gut receptors. Understanding the defence mechanisms of insects against B. thuringiensis toxicity may lead to improved pest management methods.