J. Econ. Entomol. 74: 314--3 18 ( 1981} When a small stream was treated at 0.5 ppm/15 min (l3°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. Varieties of Bacillus thuringiensis Berliner which are active against lepidopterans were shown to be relatively ineffective against mosquitoes (Hall et al. 1977) and black flies (Lacey and Mull a 1977). Consequently, this bacterium had been considered to have little potential as a biological control agent of these medically impor tant insects. The discovery of a new strain of B. thurin giensis, however, and the demonstration of its high pathogenicity to mosquitoes renewed interest in this bac terium (Goldberg and Margalit 1977) . The strain was determined to be a new serotype (H-14) and was de scribed as B. t. var. israelensis (de Barjac 1978). The laboratory effectiveness of this new variety as a black fly larvicide was subsequently demonstrated against Cnephia, Prosimulium, and Simulium spp., including the West African vector of onchocerciasis S. damnosum Theobald s. I. (Undeen and Berl 1979, Undeen and Nagel 1978). Our laboratory studies have shown that the effectiveness of the bacterium as a larvicide can be influenced by several factors, such as the species and in star of black fly, temperature, formulation, and prod uct manufacturer (Molloy et al. 1981). Reported here are results of a field trial with B. t. var . israelensis against Simulium spp. in the Adirondack Mountains of New York. Our primary objectives were: to determine the effective carry of the treatment (i.e., the distance downstream in which the bacteria continued to produce high mortality); to examine instar suscepti bility; and to monitor the effect of the treatment on non targt insect populations.