The fate and persistence of the mosquitocidal bacterium, Bacillus sphaericus, in dairy wastewater lagoons was evaluated in conjunction with trials of its larvicidal efficacy against Culex stigmatosoma. Two commercial formulations, BSP-2 (at 4.48 kg/ha) and ABG-6184 (at 2.24 kg/ha) gave about 90% reduction for up to 4 weeks, although surface water lost its insecticidal activity by 3 days posttreatment. Spores settled to the bottom within 3 days of treatment, but could be recovered in surface water after reflooding. Spore concentrations in bottom water varied widely, yet insecticidal activity remained high for from 3 days (BSP-2 at 4.48 kg/ha) to 2 weeks (ABG-6184 at 2.24 kg/ha). Spores persisted in the mud throughout the study period. These results indicate the extended control obtained was due primarily to the ingestion of spores from bottom water and mud by larvae, which routinely inhabit the shallow areas toward the edge of the pond and browse at the pond bottom.