The mortality ofAedes aegypti(L.) larvae treated withBacillus thuringiensisBerliner var.israelensiswas reduced by tannic acid at concentrations as low as 0.25 mM (425 mg/liter). The LC50's forB. thuringiensisvar.israelensistreatments were 45 ng/ml in deionized water and 233 ng/ml in 1.5 mM tannic acid. Compared with exposure of the test larvae to the bacteria immediately upon its addition to the tannic acid, soaking the bacteria in tannic acid for 48 h before the exposure of the test insects resulted in a significantly lower LC50value of 140 ng/ml and steeper log-probit regression slope. Although tannic acid lowers the gut pH ofA. aegyptilarvae, it is evident from the retention ofB. thuringiensisvar.israelensistoxicity after gut acidification in solutions of citric, hydrochloric, and phosphoric acid that this change did not mitigate the toxin's activity. Mortality of larvae exposed in leaf infusions or water collected from outdoor mosquito breeding sites was inversely correlated with the concentration of total phenolic compounds. The presence of tannins in mosquito breeding sites may require higher application rates forB. thuringiensisvar.israelensiswhen used by mosquito abatement personnel as a larvicide.