An analytical method for determining residues of dichlorvos with a limit of determination of 0.01 p.p.m. is described. I t is intended for use by those laboratories which carry out residue analysis for regulatory purposes, with an emphasis on speed, specificity and sensitivity.Residues of dichlorvos are extracted with either dichloromethane or ethyl acetate, the extracts are cleaned up by steam distillation and the concentration of insecticide is determined by gasliquid Chromatography in which one of three selective detectors, the flame-photometric, "thermionic" or electron-capture detector, is used.THE analysis for traces of dichlorvos (00-dimethyl-0-2,2-dchlorovinyl phosphate, DDVP, Vaponat) has been approached in several different ways. For exarqple, the determination of total organophosphorus has been carried out by Laws and Webleyl and by Saliman.2 Colorimetric analysis methods were developed by Geiger and Furer,3 with alkaline resorcinol as reagent, by hug he^,^ who used 2,4-dinitrophenylhydrazine and by Mitsui, Osaki, Kumano and S a n ~, ~ who used alkaline acetone. Sun and Johnson6s7 have taken advantage of the volatility of dichlorvos to devise a specific bioassay method. Cholinesterase inhibition procedures involving several different methods of determination of substrate have been described by Giang, Smith and Hall,* and by P ~r t e r , ~ and automated methods by Ott and Guntherlo * Method submitted for discussion t o the Commission on Residue Analysis, Pesticides Section, Applied t "Vapona" is a Shell Trade Mark.