The control of black fly larvae using 2 aqueous suspension formulations of Bacillus thuringiensis, serotype H-14 was evaluated in 6 trials on small and large sized streams in New Brunswick. Applications equivalent to 25 ppm (Vectobac-12AS) or 12.5 ppm (Vectobac-24AS) over a 1-min period resulted in 100% control of black fly larvae up to 3 km downstream of the application point. Over the 6 trials, effective carry was seen to increase with increased stream discharge which varied from as low as 0.8 m3/min in the small stream to 108.6 m3/min in the large stream.