Four application rates of Bacillus sphaericus strain 2362 were tested for efficacy in septic ditches against Culex quinquefasciatus 2nd-4th instar larvae. Trials were conducted over a 2-year period. In 1987, all dosages applied to dairy effluent ditches resulted in substantial reductions after 48 h. Differences among dosages did not differ significantly (P greater than or equal to 0.05). After 5 days, mosquito larval numbers increased with dosage rates of 0.6, 0.9 and 1.2 liters/ha. Residual control was maintained, however, at 2.4 liters/ha for 17 days. In 1988, B. sphaericus applied to domestic sewage effluent ditches at rates of 1.2 liters/ha, and 2.4 liters/ha induced greater than 79% suppression for a period of 15 days, and the 1.2 liters/ha rate induced greater than 79% suppression for a period of 20 days. A rate of 0.9 liters/ha provided good to excellent control (greater than 88%) for a period of 10 days. A rate of 0.6 liters/ha produced less than 50% suppression after 48 h.