Bacillus thuringiensis var. israelensis (B.t.i.), HD-968-S-1983, and a primary powder of Bacillus sphaericus 2362 were assayed against Mansonia titillans and Ma. dyari. The susceptibility of the 2 species to the individual bacterial toxins was similar. The B.t.i. LC50 values were 50.5 micrograms/liter for Ma. dyari and 54.7 micrograms/liter for Ma. titillans; the B. sphaericus LC50 values were 339.0 micrograms/liter and 347.2 micrograms/liter, respectively. The log dose-probit mortality lines were parallel for both target species. The potency of B. sphaericus relative to B.t.i. was 0.15 for Ma. dyari and 0.16 for Ma. titillans.