Three tests were conducted during July 17 to October 30, 1998 to study the impact of two mosquitocidal microbial agents on mosquito larvae and their contribution to bacterial flora in aquatic microcosms. Formulations of Bacillus thuringiensis subsp. israelensis (Bti) and Bacillus sphaericus strain 2362 (Bsph) were applied at various rates to outdoor tubs enriched with rabbit pellets and filled with irrigation water from a reservoir. Mosquito larvae were effectively controlled by all treatments; the magnitude of initial and persistent control depended on materials and dosages applied. Bacterial flora were assessed in the irrigation water as well as water in the enriched tubs before and after treatment with the microbial agents. The irrigation water contained 800-1000 total bacterial cells/ml. The populations of total bacteria and spore formers peaked on day 3 after enriching and filling the tubs, then declined progressively to the low levels at the end of the tests. After treatment, the numbers of Bti and Bsph spores in treated tubs prevailed at a dosage-dependent manner, their populations peaked at three hours after treatment, and declined progressively thereafter. The contribution of Bti and Bsph spores to the total bacterial flora was negligible but significant to the counts of spore-forming bacteria. The gram-negative bacteria made up more than 80% of the total bacterial flora during the test periods; and, of these, gram-negative rods constituted the greatest proportion, gradually increasing from the time of flooding to the end of the tests. Gram-negative cocci also occurred in relatively great proportion, but showed a reverse trend as compared with the gram-negative rods, declining gradually from pretreatment to the end of the tests. Gram-positive rods (spore formers), including Bti and Bsph, occurred in low numbers in all the tests but increased slightly in treated tubs due to the addition of Bti and Bsph spores. Gram-positive cocci occurred occasionally in some water samples.