The 2297 isolate (serotype 25) of Bacillus sphaericus was bioassayed in the laboratory against 8 species of mosquitoes from 3 subfamilies. The most susceptible species were in the genus Culex and the least susceptible were the Aedes spp. and Toxorhynchites r. rutilus. Primary powders of the 2297 and 2362 (serotype 5a5b) isolates were evaluated in the field in natural and simulated habitats against Culex spp. The larvicidal activity of the two isolates was similar, with longer residual activity observed for both preparations in shaded shallow clear water. Larvicidal activity was curtailed in organically enriched and unshaded habitats. Isolte 2297 provided effective control for at least 1 week in an organically enriched habitat and for over 5 weeks in clear water in a shaded habitat when applied at the rate of 0.25 kg/ha.