Aspergillus fumigatus ATCC 28282 was found to be capable of growth on 4-ethylphenol as its sole carbon and energy source. A pathway for the metabolism of this compound has been proposed. The initial step involves hydroxylation of the methylene group of 4-ethylphenol to form 1-(4'-hydroxyphenyl)ethanol, followed by oxidation to 4-hydroxyacetophenone. The hydroxylase was NADPH and oxygen dependent, which is a characteristic of a monooxygenase type of enzyme. The 1-(4'-hydroxyphenyl)ethanol isolated from growth medium was a racemic mixture of R-(+) and S-(-) enantiomers. 4-Hydroxyacetophenone undergoes an NADPH-dependent Baeyer-Villiger type of oxygenation to give 4-hydroxyphenyl acetate, which is hydrolyzed to form hydroquinone (1,4-dihydroxybenzene). Hydroxylation of hydroquinone by an NADPH-dependent enzyme produces 1,2,4-trihydroxybenzene, the ring fission substrate, which is cleaved by ortho fission to form maleylacetate. The pathway was elucidated by various kinds of investigations. Analysis of culture medium sampled during growth on 4-ethylphenol revealed the transient appearance of 1-(4'-hydroxyphenyl)ethanol, 4-hydroxyacetophenone, and hydroquinone. Cells grown on 4-ethylphenol were able to oxidize all of these compounds immediately, whereas oxidation by succinate-grown cells showed a lag period. Extracts prepared from cells grown on 4-ethylphenol contained enzyme activities for all of the proposed steps. Apart from a low level of esterase activity towards 4-hydroxyphenyl acetate, extracts prepared from cells grown on succinate did not contain any of these enzyme activities.