Bacillus thuringiensis serotype H-14 (Bti) was tested in small rice plots and in commercial rice fields. Application rates of 0.5 and 1.0 kg/ha in small plots resulted in 100% mortality of Psorophora columbiae at 24 hr posttreatment and slight residual activity 48 hr posttreatment (i.e., 13 and 27% respectively). It was also applied by airplane against 2 species at 1.0 kg/ha in 2 large field trials. Larval mortality of Ps. columbiae was 69 and 89% after 24 hr exposure in cups. Mixed natural populations of Anopheles quadrimaculatus larvae (mainly 1st and 2nd stages) were also reduced by 97%.