Bacillus sphaericus strains SSII-I, 1404-9 and 1593-4 were applied to field populations Psorophora columbiae larvae were treated with strain 1593-4. In the 3 experiments larval populations were reduced by nearly 90% when the bacterial suspensions were applied in 3.1 x 104 to 1.9 x 103 dilutions of whole preparation. Bite count data suggest a reduction in adult emergence due to the treatments.