Residues of Abate® (O, O-dimethyl phosphorothioate O,O-diester with 4,4′-thiodiphenol) were determined in larvae of the southern house mosquito, Culex pipiens quinquefascialus Say, and in larvicide suspensions by gas chromatography at 260°C. The Melpar flame photometric detector, which responds with high specificity to phosphorus compounds, was used in conjunction with a water-cooled adapter designed by the authors to permit high-temperature operation of the detector. Recoveries of Abate from freshly prepared aqueous suspensions were essentially 100%, and those from groups of 100–500 mosquito larvae spiked with the insecticide before extraction averaged 99% Essentially no Abate « 2%) was lost from suspensions (0.10–10 ppm) after it stood for 16 days in open containers. A high proportion (42–57%) of the residue was strongly attached to the surfaces of the containers. Larvae exposed to such suspensions for 1–2 hours (100% mortality) contained 3–100 ppm of Abate. Glass beads coated with Abate and allowed to stand in water produced concentrations of 0.029±0.002 ppm in the supernatant at 1–16 days.