The presence of extensive aquatic vegetative growth (Potamogeton crispus and P. pectinatus) had little effect on reducing the larvicidal activity of Bacillus thuringiensis var. israelensis (Bti) against Simulium vittatum. Following a 35 min exposure to 3.10 ppm (98,800 spores/ml) suspension of Bti, the reduction in S. vittatum larvae over a 96-hr period ranged from 27 to 92% along the entire 312 m length of the test stream.