Sublethal effects of three pesticides including atrazine (triazine herbicide), DDT (organochlorinated insecticide), and chlorpyrifos (organophosphate insecticide) on acetylcholinesterase (AChE), general esterase (GE), glutathione S-transferase (GST), and cytochrome P450 monooxygenase (P450) activities were evaluated in the aquatic midge Chironomus tentans. Exposures of midges to atrazine at 30 and 150 micrograms per liter (microg/L) for 20 d (i.e., from the first- to fourth-instar larvae) enhanced P450 O-deethylation activity by 12.5- and 15.5-fold, respectively, but did not significantly change AChE, GST, and GE activities. Similar exposures to DDT at 0.01 and 0.05 microg/L did not significantly affect AChE, GE, and P450 activities; however, DDT at 0.05 microg/L enhanced GST activity toward the substrate 1-chloro-2, 4-dinitrobenzene by 33.6%. Exposures of midges to chlorpyrifos at 0.10 microg/L for 20 d reduced AChE activity by 59.8%, and GE activities toward the substrates alpha-naphthyl acetate and beta-naphthyl acetate by 30.7 and 48.8%, respectively. The reduced GE activities appear to be due to the inhibition of several esterases, particularly the one with a slow migration, by chlorpyrifos as demonstrated by non-denaturing polyacrylamide gel electrophoresis. Furthermore, exposure of midges to chlorpyrifos at 0.10 microg/L for 20 d enhanced the P450 O-deethylation activity by 3.3-fold although no significant effect was observed at 0.02 microg/L for the same enzyme. These results provide insights into the sublethal effects of these commonly detected pesticides in aquatic environments on important enzymes in aquatic midges.