The fungal metabolism of tert-butylphenyl diphenyl phosphate (BPDP) was studied. Cunninghamella elegans was incubated with BPDP for 7 days, and the metabolites formed were separated by thin-layer, gas-liquid, or high-pressure liquid chromatography and identified by 1H nuclear magnetic resonance and mass spectral techniques. C. elegans metabolized BPDP predominantly at the tert-butyl moiety to form the carboxylic acid 4-(2-carboxy-2-propyl)triphenyl phosphate. In addition, 4-hydroxy-4'-(2-carboxy-2-propyl)triphenyl phosphate, triphenyl phosphate, diphenyl phosphate, 4-(2-carboxy-2-propyl)diphenyl phosphate, 2-(4-hydroxyphenyl)-2-methyl propionic acid, and phenol were detected. Similar metabolites were found in the 28 fungal cultures which were examined for their ability to metabolize BPDP. Experiments with [14C]BPDP indicated that C. elegans metabolized 70% of the BPDP after 7 days and that the ratio of organic-soluble metabolites to water-soluble metabolites was 8:2. The results indicate that fungi preferentially oxidize BPDP at the alkyl side chain and at the aromatic rings to form hydroxylated derivatives. The trace levels of mono- and diaryl metabolites and the low level of phosphotriesterase activity measured in C. elegans indicate that phosphatase cleavage is a minor pathway for fungal metabolism of BPDP.