In the laboratory, 5 organophosphorous (OP) and 4 new synthetic pyrethroid insecticides were bioassayed against 4th instar field populations of Chironomus decorus (Johannsen), Dicrotendipes californicus (Johannsen), Cricotopus bicinctus (Meigen), Cricotopus sylvestris (Fabr.), and Tanypus grodhausi Sublette collected from 2 flood control channel systems in southern California. The OPs included chlorpyrifos, temephos, fenthion, malathion, and phenthoate, while the pyrethroids were FMC-45498 or decamethrin, FMC-45497, FMC-35171, and SD-43775 or fenvalerate. On Coyote Creek midges, malathion was the most active OP against Cricotopus spp. (LC90=0.35 ppm) and C. decorus (LC90 =0.21 ppm), while D. californicus was most susceptible to chlorpyrifos (LC90 = 0.16 ppm) and least to malathion (LC90 = 1.42 ppm). The LC90 values of chlorpyrifos and temephos against T. grodhausi were 0.0039 and 0.31 ppm, respectively. In general, the Coyote Creek midges were resistant to the OPs tested showing the highest resistance to temephos. However, these species were highly susceptible to the 4 pyrethroids. FMC-45498 and FMC-45497 were the 2 most active with LC90 values ranging from 0.00013-0.012 ppm. The San Jose Creek species of Cricotopus and D. californicus were far more susceptible to the organophosphates (LC90 range of 0.0009-0.11 ppm) than the same species in the Coyote Creek. FMC-45498 and FMC-45497 were also highly toxic to these midges showing much superior activity than chlorpyrifos, the most active OP insecticide against midges in the San Jose Creek.