Except for moderate mortality among filter-feeding chironomids, Rheotanytarsus spp., the results of 10 field trials with Bacillus thuringiensis var. israelensis (B.t.i.) indicated a wide margin of safety to the chironomid community and other stream nontarget insects. Mayflies, caddisflies and 2 other types of chironomids, i.e., tube-dwelling (Chironominae) and surface-dwelling, caseless larvae (mix of Chironominae, Diamesinae, Orthocladiinae, Tanypodinae), did not appear to be affected. The susceptibility of filter-feeding chironomids was considerably less than black flies; for example, 4 months of data collected during an operational black fly control program indicated a mean (+/- 95% CI) mortality among Rheotanytarsus larvae of 23 (15-32)% vs. 98 (97-99)% among black flies. Although clearly demonstrating the potential of adverse impact on filter-feeding chironomids in operational black fly programs, these trials also confirmed the narrow impact of B.t.i. on the overall stream insect community.