Bacillus thuringiensis var. israelensis (Bti) was about 85 x more active against 4-day-old larvae of Aedes aegypti in distilled water than in pond water (LC-50's of 0.7 vs. 59.7 ug/ml); also, the difference in LT-50 for 1.0 ug/ml was almost 30-fold. An increase in the concentration of pond-water-sediment resulted in a corresponding decrease in activity of Bti. About on-half of the Bti activity was bound by a sediment concentration of 2%. Bti suspended in either pond or distilled water was inactivated after 24 hours' exposure to simulated sunlight-UV. There was no apparent loss in the insecticidal activity of an aqueous suspension of Bti held at 30o + 1o C for more than 100 days. A water salinity of 0.5% had no apparent effect on the larvicidal activity of Bti, nor was activity affected in distilled or natural pond water at pH of 4.0, 6.6, and 10.0. No increase in activity of Bti due to regrowth after suspension in water was detected. Presence of food or of Bti-killed mosquito larvae increased both the rate and extent of mortality. Larval feeding depleted the total activity of a suspension of Bti; less than 1% was present after feeding by 3 serial populations of larvae. Suspensions of Bti settle in still water; differences in activity between a top and bottom sample could be detected after 1 hr of standing. All available laboratory data indicate that var. israelensis is a very promising candidate microbial insecticide for control of mosquitoes.