The effects of the molting-hormone agonistic insecticide tebufenozide on larvae of the midge Chironomus riparius Meigen were tested in two different exposure setups. After static contamination of first-instar larvae the NOEC, LOEC, and LC50 values were 13.2, 17.4, and 21.14 microg/L, respectively. Semistatic exposure of fourth-instar larvae revealed a lower susceptibility of elder larvae (NOEC 30 microg/L, LOEC 60 microg/L, and LC50 81.94 microg/L). In both cases mortality was not immediate; the effects were postponed and almost exclusively linked to the processes of pupation and emergence. Pupal mortality in the semistatic exposure scheme was twice as high in males as in females during a 100 microg/L treatment. This sex-specific effect probably resulted from the endocrine activity of tebufenozide. Its detection underlines the suitability of C. riparius as a model organism for investigating effects of endocrine-disrupting chemicals in aquatic insects.