Bacillus thuringiensis var. israelensis (B.t.i.), was applied for control of Psorophora columbiae larvae to rice fields being flooded for a second rice crop in August and September 1980. Rates of 2 commercial wettable powder formulations applied in a water suspension ranged from 0.25 kg/ha to 0.6 kg/ha. Active ingredients, expressed as the potency value (International Toxic Units), ranged from 8.1 x 108 to 1.9 x 109/ha. Three applications by airplane resulted in 91, 96 and 99% reductions of very dense (4.5 to 18.6 larvae/dip) larval counts as compared to counts taken 24 hr prior to treatment. The B.t.i. was also adhered to rice hulls and applied prior to flooding the rice fields or at the time of flooding, and at the same rates as applied in water suspension. The rice hull tests resulted in inadequate control.