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A pond mesocosm study was conducted in a central Minnesota wetland to evaluate the potential toxicity of the microbially-derived insecticide Bacillus thuringiensis var. israelensis (B.t.i.) to chironomids. B.t.i. was applied as VectoBac® G to mesocosms on two occasions (21 d apart) at five rates (0.3X, 1X, 2.5X, 5X, 10X) with three replicate mesocosms per rate. The 1X rate (9 kg/ha) was that operationally used by the Minneapolis-St. Paul Metropolitan Mosquito Control District for early summer mosquito control. Chironomid abundances following B.t.i. treatment were compared to abundances in untreated control mesocosms. The abundance of Chironomidae larvae was significantly reduced at the 10X treatment 4 d after the first B.t.i. application. Chironomid abundance was also reduced after the second application with 10X, but showed strong signs of recovery within 32 d. Chironominae, the numerically dominant subfamily within the Chironomidae, showed a similar response. The abundance of Orthocladiinae larvae was significantly reduced at both the 10X and 5X treatments, whereas the Tanypodinae appeared unaffected by all B.t.i. treatments. Of the two tribes comprising the Chironominae, the Chironomini displayed a response very similar to that of its parent subfamily, although reductions in abundance were not statistically significant. The tribe was dominated by Dicrotendipes, Einfeldia, and Endochironomus, none of which were significantly reduced following either 10X application. The second tribe, the Tanytarsini, were slightly more susceptible to B.t.i. than the Chironomini, displaying significant reductions in abundance after both 10X applications. The Tanytarsini were dominated by Paratanytarsus, which were reduced by 91% 4 d after both 10X B.t.i. applications. Tanytarsini and Chironomini were also reduced in abundance (by 83 and 75%, respectively) at the 5X treatment, but reductions were not statistically significant. Regressions of larval chironomid abundance versus B.t.i. treatment rate indicated that the B.t.i. rates required to reduce chironomid abundance by 25, 50, and 75% were 1.5–2.0X, 2.1–3.3X, and 3.5–11.0X, respectively. Emergence of adult Chironomidae was significantly reduced at the 10X B.t.i. treatment, but not at 5X. The same trend was observed for the Chironominae, which comprised 82% of the family, but not for Orthocladiinae and Tanypodinae. Emergence of Ceratopogonidae and Chaoboridae was unaffected by all B.t.i. treatments.