Suspensions of Bacillus thuringiensis var. israelensis spores and crystals were tested as a Simuliid larvicide in small Newfoundland streams. One minute dosages with initial concentrations of 1 x 105 viable cells/ml caused up to 100% mortality in larval Simuliids. The flow rate of the stream appeared to be a major variable in determining the maximum distance downstream from the done site at which the treatment was still effective.