Defines a gas chromatographic method for determining residues of organophosphorus pesticides in animal feeding stuffs. Suitable stuffs containing residues of one or more of organophosphorus pesticide; fonofos, parathion, ethion, dimethoate, diazon, azinphos-ethyl, bromophos, chlorpyrifos, carbophenothion, chlorpyrifos-methyl, azinphos-methyl, malathion, methidathion, pirimiphos-ethyl, parathion-methyl and pirimiphos-methyl.