Six diked dredged material disposal areas (7-yr-old) in coastal North Carolina were treated with controlled-release pellets of chlorpyrifos (Dursban 10CR) at the recommended rate, and the water and underlying soil (dredged material) were analyzed by GLC (gas-liquid chromatography) to determine the chlorpyrifos residue levels at ca. monthly intervals pre and posttreatment. Residue levels were usually much greater in the soil than in the water. The residue levels were extremely variable and the probable reasons are discussed. Overall, a minimum of 0.7 ppb of chlorpyrifos in the water of the field sites was needed for control of Aedes taeniorhynchus. Within 7 months after treatment, the chlorpyrifos residues were generally too low for practical mosquito control. In standardized laboratory bioassays using Ae. taeniorhynchus larvae, the LC50 and LC90 of chlorpyrifos residues in water from dredged material disposal areas were 0.69 and 15.4 ppb, respectively, while the LC50 and LC90 using technical chlorpyrifos and distilled water were much lower (0.09 and 0.21 ppb, respectively).