In the laboratory, 4 organophosphorous (OP) and 6 new synthetic pyrethroid insecticides were tested against field-collected 4th instars of Glyptotendipes paripes Edwards, Chironomus decorus Johannsen, C. crassicaudatus Malloch, Goeldichironomus holoprasinus (Goeldi), and Tanytarsus spp. midges. The OPs tested were chlorpyrifos, temephos, fenthion, and malathion, while the pyrethroids included FMC-30980, FMC-33297, FMC-35171, FMC-45497, FMC, 45499, and FMC-52703. All the pyrethroids showed a very high level of activity against G. paripes, C. decorus and C. crassicaudatus with LC90 values ranging from 0.00026-0.013 ppm; G. paripes was the most susceptible of the 3 species. FMC-52703 proved the most toxic to these species. Among the 4 OPs, malathion was the most active against G. paripes (LC90=0.0079 ppm), followed by temephos, chlorpyrifos, and fenthion. Chironomus decorus and C. crassicaudatus were generally tolerant to the OPs (LC90=0.1-0.48 ppm). Goeldichironomus holoprasinus and Tanytarsus spp. were also tolerant to the OPs, showing the most tolerance to chlorpyrifos (LC90 =1.2 and 2.4 ppm, respectively). However, these 2 midge groups were very highly susceptible to the pyrethroids. The LC90 level of FMC-52703 against G. holoprasinus was as low as 0.0004 ppm and that against Tanytarsus spp. was 0.000019 ppm.