The microbial control agents Bacillus thuringiensis (H-14) and B. sphaericus were evaluated in laboratory and field against Psorophora columbiae. Bacillus sphaericus strain 2362 was also tested in the field against Aedes melanimon. Psorophora columbiae was slightly more susceptible than Culex quinquefasciatus to active strains of B. sphaericus. The LC90 for active strains ranged from 0.013 to 0.069 mg/liter. In field trials, aqueous suspensions of primary powder of B. sphaericus 2362 and 1593 yielded 98-99% reduction in larvae at the rates of 0.1 to 0.25 lb/acre of the primary powder. Granular formulations of Bt (H-14) were evaluated against Ps. columbiae, yielding 96-99% control of larvae at rates ranging from 1 to 10 lb/acre of the granules, depending on the potency and type of the formulations. Aedes melanimon was slightly less susceptible than Ps. columbiae to B. sphaericus 2362. In warmer water a rate of 0.25 lb/acre of the primary powder yielded 88% control, while this same rate in cool weather yielded only 4% reduction. A rate of 0.5 lb/acre of the primary powder was needed to obtain 94% control of larvae in cool weather.