The latest approach to control of midge larvae in drinking-water supplies is suppression of the planktonic 1st-stage larvae, by using 2 disinfectants, chloramine and copper sulfate. The median lethal concentration for 24-h exposure of the 1st-stage larvae of Chironomus luridus to chloramine and copper sulfate individually was 0.51 and 0.38 mg/liter, respectively. The increase of copper sulfate to 0.5 mg of copper per liter to water containing chloramine (0.5 mg/liter) created a synergistic reaction that resulted in 96% (+/-8% SD) mortality of the planktonic larvae. This treatment may serve as an effective control of 1st-stage larvae in municipal drinking-water supplies.