A section of forest stream was treated withBacillusthuringiensisBerliner var.kurstaki(B.t.k.) at 10 × the expected environmental concentration (200 IU/mL) to determine effects on the macroinvertebrate community. Invertebrate drift density increased slightly (approximately 2-fold over pre-treatment densities), but only during the 0.5-h application and only at the site 10 m below the application point. There were no significant changes in taxonomic richness of benthic invertebrates after the application, but there were short-term alterations in community structure at the treated site after the application, as measured by the χ2dissimilarity index. In 11 of 12 benthic taxa for which there were sufficient data, changes in abundance after the application were not significant compared with changes in abundance at the reference site. The stoneflyLeuctratenuis(Pictet) was significantly reduced (~70%) at the treated site 4 d after the application, and abundance of this stonefly remained considerably lower, but not significantly different, from the reference site for at least 18 d. A follow-up study demonstrated that under laboratory conditions,B.t.k. on leaf material was not toxic toL.tenuis. TheB.t.k. application had no significant effect on the growth or survival of caged caddisfly larvae,Pycnopsycheguttifer(Walker), in the treated stream. Implications of results from this field experiment are discussed.