A field trial for the control of Psorophora columbiae with Bacillus thuringiensis var. israelensis was conducted in small rice plots. Forty-eight hour mortality increased from 61.5% at a dose of .44 parts per million (ppm) to 100% at a dose of 4.4 ppm when 2nd stage larvae were placed in the plots immediately after treatment. When 3rd and 4th stage larvae were placed in the plots 48 hr after treatment, 72 hr mortality varied from 26.3% at a dose of .44 ppm to 75% at a dose of 8.0 ppm.