The metabolism and detoxication of 14C-(ethoxy) and 14C-(ring)-Dyfonate were demonstrated with isolated soil fungi. Quantitative and qualitative differences in the production of insecticide metabolites were observed. Among the nine fungal species tested, Mucor plumbeus and Rhizopus arrhizus were most active in degrading the insecticide, while Penicillium notatum was the least active. This activity was evidenced by the production of increasing amounts of water-soluble 14C-compounds in the culture media and the disappearance (degradation) of the originally applied insecticide from the fungal cultures. The water-soluble metabolites were non-toxic to mosquito larvae. Hexane-extractable compounds from cultures of the most active species were non-toxic to fruit flies, while those from cultures of the less active fungal species were insecticidal. Among the metabolites produced and detected by thin-layer chromatography were primarily dyfoxon, ethylethoxyphosphonothioic acid, ethylethoxyphosphonic acid, methyl phenyl sulfoxide, and methyl phenyl sulfone. With Aspergillus niger and 14C-(ring)-Dyfonate, most of the radiocarbon recovered from the TLC-plates was associated with an unknown compound. Production of the metabolites by the fungi was a function of the live mycelia, followed by excretion of water-soluble 14C-compounds into the culture media.