A method is reported for the simultaneous determination of coumaphos (o,o-diethyl o(3-chloro-4-methyl-2-oxo-2H-1-benzopyran-7-yl)phosphorothioate) and its oxygen analog, Coroxon, in eggs in the ppb range. The residues are extracted with acetone and chloroform. The extract is purified by liquid-liquid partition followed by column chromatography and then by 2-dimensional thin layer chromatography. The fluorescence is measured directly on the chromatogram.