the biodegradation of fenitrothion (0,0-dimethyl-0-(3-methyl-4-nitrophenyl)- phosphorothioate and fenitrooxon (0,0-dimethyl-0-(3-methyl-4-nitrophenyl)phosphate by the soil fungus trichoderma viride pers. has been investigated. test compounds were incubated with pure cultures of the fungus. the liquid cultures were extracted with ethyl acetate and the residues analysed by gas chromatography. fenitrothion, fenitrooxon and their hydrolysis product, 3-methyl-4-nitrophenol, were identified in the fenitrothion degradation experiments, while only the hydrolysis product and unreacted starting material was recovered from fenitrooxon test cultures. a clean-up procedure involving ion-exchange chromatography using amberlyst a-21 resin was developed to facilitate residue analysis. evidence indicated that the fungal degradation of fenitrothion involves both oxidative and hydrolytic pathways. furthermore, the evidence suggested that the 3-methyl-4-nitrophenol hydrolysis product is cometabolised by the fungus.