Two preparations of the microbial control agent Bacillus sphaericus (Strain 2362, 1F-117) were evaluated in organically enriched replicated field ponds for assessment of their efficacy and short-term persistence against mosquito larvae. Spores of the cream and powder preparations settled quickly and accumulated in the mud zone out of reach of feeding mosquito larvae. Both cream and powder formulations yielded 90% + initial control of larvae at the rate of 0.2 Ib/acre (0.22 kg/ha) in ponds without or with low level of enrichment. In clear water and low level of pollution, this rate of application yielded good control of larvae for about 3 weeks. This extended level of control with this agent is an improvement over currently available larvicides. Extent of larval control was not high in ponds with high level of enrichment. In all ponds, predaceous insects peaked or started to increase in about 2-3 weeks posttreatment exerting regulatory force on mosquito larvae with the exception of the highly enriched ponds. In the highly enriched ponds, the predators built up to high levels, but they could not cope with the extremely heavy populations of mosquito larvae. Bacillus sphaericus 2362 had no adverse effects on dominant macroinvertebrate fauna, especially predaceous insects. The use of 2362 and other active strains therefore offers a good potential for use in integrated mosquito control program.