FENITROTHION, dimethyl 3-methyl-4-nitrophenyl phosphorothionate, the active ingrcdient of the commercial formulation, Sumithion, has a low mammalian toxicity, viz., acute oral LD,, to rats, 673 mg per kg, and is effective against a wide range of insects.l It has shown promise for the control of cocoa capsids, Didantiella theobvorna, and there was a need for a method for determining residues of the insecticide in treated cocoa beans at harvest. Dawson, Donegan and Thain2 described a gas - liquid chromatographic method for determining residues of fenitrothion and related compounds in cocoa beans, after extracting them into benzene and purifying by solvent partitioning. Kovac and Sohler3 extracted fenitrothion residues from fruits and vegetables with light petroleum and separated them by thin-layer chromatography, with subsequent ultraviolet absorptiometry of the hydrolysed product, 3-methyl-4-nitrophenolate. Similar methods were used for determining traces of fenitrothion in milk,* bananas6 and cocoa beans6 The colorimetric method, developed by Averell and Norris' for determining residues of parathion (diethyl 4-nitrophenyl phosphorothionate) , may be adapted for determining fenitrothion. The method depends on reduction of the insecticide to the corresponding aromatic amine and the formation of a coloured compound by diazotisation and coupling with N- 1 -naphthylethylenediamine. When applied to cocoa beans, it suffers from interference by plant pigments and fat, which comprise about 50 per cent. of the bean.8 To overcome these difficulties, the clean-up procedure and the effect of several variables on the development of the colour were investigated. In the method described below, the insecticide is extracted into chloroform. The extract is decolorised with activated charcoal, and the fat removed from the hot reduced solution by adding paraffin which collects the oily substance and the zinc powder as a solid mass on cooling. The final colour is developed in a hydrochloric acid - isopropanol solution, and reaches the approximate maximum intensity 1 hour after adding the coupling reagent.