The development and screening of floating-type bait formulations designed to improve the activity of bacterial toxins against larval Anopheles is described. Floating and spreading abilities of carrier particles (wheat flour) were compared using corn oil, lecithin, and two products yielding surface films on water (Arosurf and Liparol). Mixtures containing 1 or 5% Arosurf showed the best spreading abilities on a water surface, but strongly inhibited the ingestion of wheat flour by Anopheles albimanus larvae. Corn oil and lecithin improved spreading satisfactorily at a concentration of 5% and inhibited larval feeding by only 6-25%. To select a suitable concentration of active ingredient in formulations, Bacillus thuringiensis var. israelensis (B.t.i.) primary powder in concentrations ranging from 0.01 to 0.5% was mixed with wheat flour/corn oil mixtures and tested in quantities exceeding the gut volumes of treated larvae. Complete mortality was obtained with concentrations of 0.1% (Anopheles stephensi), 0.2% (An. albimanus), or 0.3% (An. quadrimaculatus) B.t.i. When in 175-liter containers the activity of formulations (5% corn oil, 0.2% B.t.i.) and of toxin suspensions was compared by conventional dosage-mortality regression, formulations were more active by a factor of 68 against An. stephensi, 39 against An. albimanus and 67 against An. quadrimaculatus.