The oxygen analog of ronnel, O,O-dimethyl O-(2,4,5-trichlorophenyl) phosphate, was identified in the extracts of ronnel-treated Madeira cockroaches, Leucophaea maderae (F.). The extracts were analyzed by cholinesterase assay for inhibitors on intact paper chromatograms developed by normal and reverse phase solvent systems. It was established that the phosphate was produced by in vivo oxidation of ronnel, O,O-dimethyl O-(2,4,5-trichlorophenyl) phosphorothioate, in the cockroaches and not during subsequent analytical steps or by administration of the phosphate as a contaminant of technical ronnel. The enzyme method is highly sensitive, easily detecting 0.01 µg. or less of the phosphate and when combined with several chromatographic systems should be widely applicable in detecting and identifying anticholinesterase metabolites formed in biological systems.