Bacillus sphaericus is a spore-forming aerobic bacterium, several strains of which are pathogenic for mosquito larvae. During sporulation, the most active strains produce a crystal toxin with a high degree of larvicidal activity. The toxin is composed of two proteins of 51.4 and 41.9 kDa, which are encoded by highly conserved chromosomal genes. After B. sphaericus is ingested, these proteins are released in the larva's midgut, and, in susceptible mosquito species, bind to a specific receptor present on midgut brush-border membranes. The resulting damages to the midgut cells leads to the mosquitoes' death. During vegetative growth, some B. sphaericus strains also synthesize mosquito larvicidal proteins of 100 and 30.8 kDa (Mtx toxins), the mode of action of which is still unknown. The mechanism of acquisition of the recessive mosquito resistance to the crystal toxin varies with selection conditions.