Dursban® insecticide (O,O-diethyl O-3,5,6-trichloro-2-pyridyl phosphorothioate) has been evaluated in large-scale field tests. The persistence and gross effects on the biotic community found in a salt marsh resulting from residues of Dursban were investigated. An aerial application of 0.025 1b of Dursban per acre controlled adult salt marsh mosquitoes, Aedes sollicitans (Walker), and had no obvious adverse effects on caged and natural specimens of shrimp, minnows, crabs, fish, and birds. Increasing the dosage level to 0.05 lb per acre resulted in a reduction in numbers of marsh-feeding brown shrimp, and moderate mortality was caused among the small fish and minnows. Larger fish and blue crabs appeared to be unharmed, even when confined in the treated area for 21 days. Gas chromatography with electron-capture detection was used to study the persistence of Dursban in water, silt, and oysters collected from the test area following application. Residues of Dursban in water samples of the treated marsh averaged 0.0038 ppm (0.0022-0.0064) I hour after application of 0.025 lb per acre and were not detectable 7 days after treatment. In the 0.05-lb-per-acre test, water samples contained an average of 0.0054 ppm (0.0037-0.0082) 1 hour after treatment, which decreased to 0.0004 ppm (0.0002-0.0008) after I day and were not detectable at 13 and 21 days after treatment. Silt samples collected from 7 locations at 7 and 13 days showed no detectable residues of Dursban. Oysters contained from <0.005 to 0.042 ppm Dursban I day after treatment. Residues disappeared rapidly from the oysters, decreasing to 0.006 ppm 2 days after treatment, and were not detectable at 3 days post-treatment.