A new microgel droplet formulation of Bacillus sphaericus 1593 M (Biocide-S) was tested at 2 rates against culicine vectors of Japanese encephalitis and Anopheles subpictus. A single application just after transplantation of rice seedlings prevented the buildup of anopheline as well as culicine populations, and gave 83-100% reduction of pupal density at the lower rate (2.2 kg/ha) and 87-100% reduction at the higher dosage (4.3 kg/ha) for at least 5 weeks. During the last 3 wk before harvest, density was naturally very low and the efficacy of the treatment could not be assessed. This formulation has a potential for control of riceland mosquitoes, but a suitable delivery system is required before it can be considered for practical control.