Representative aquatic insects, larvae of Simuliidae, Chironomidae, Trichoptera, Megaloptera, and nymphs of Ephemeroptera and Plecoptera, were tested for susceptibility to continuous exposure to Bacillus thuringiensis Berliner var. kurstaki at 4.3, 43, and 430 IU/mL. Only Simulium vittatum was affected, and at the highest concentration. Effects on other organisms, particularly Prosimulium fuscum/mixtum, were suggested. The lowest of the concentrations tested was twice the worst-case transitory concentration peaks expected in water from aerial forest spraying at 30 BIU/ha. Spray buffer zones around water bodies are unnecessary at this spray rate.