When a small stream was treated at 0.5 ppm/15 min (13°C) with an aqueous suspension of unformulated Bellon primary powder of Bacillus thuringiensis Berliner var. israelensis, 96, 86, 53, and 11\% reductions occurred, respectively, in larval populations of Simulium spp. black flies at 20, 180, 350, and 705 m below the treatment point. In contrast, the number of black fly larvae declined by only 5\% in the upstream control. Mortality occurred within 2–3 days of treatment, with few dead larvae remaining attached by the 5th day posttreatment. The mortality rates of larvae transported to and reared in the laboratory for 48 h posttreatment closely corresponded to the rates recorded in the stream. Significantly higher mortality rates occurred among smaller larvae. In contrast to the sharp reduction (89\%) in black fly larvae in the 20–350 m area below the treatment point, Surber samples indicated increases in mayfly (35\%), caddisfly (47\%), stonefly (75\%), chironomid (19\%), and elmid (242\%) populations. Thus, no adverse effect on any of these nontarget populations was evident following stream treatment.