Immunofluorescent staining was used with thin sections of paraffin-embedded specimens to detect the development of Bacillus thuringiensis var. israelensis and Bacillus sphaericus in the gut of mosquito larvae. The third- and fourth-instar larvae of Aedes aegypti, Anopheles maculatus, and Culex quinquefasciatus were fed either vegetative cells or spores of the bacteria. Spore germination, multiplication, and sporulation were studied in the larvae of each species. The spores of B. thuringiensis var. israelensis and B. sphaericus strain 2297 could germinate and cells could sporulate in the larval body. The vegetative cells of B. sphaericus strain 810428 were also able to produce spores in the mosquito larval gut, but the germination of spores could not be detected in the larvae. Multiplication of all bacterial species was observed after the larvae died. Growth of the bacteria in distilled water containing crude extracts of larvae made from each species was compared with that in synthetic medium (nutrient broth). They could produce spores and toxins in all the media used and the toxins had larvicidal activity against the target mosquitos Ae. aegypti, An. maculatus, and C. quinquefasciatus.