Strains (47) of 14 genera isolated from soil and woody chips were tested for their ability to oxidize dibenzofuran, a model compound for highly chlorinated dibenzofuran pollutants. Thirty strains formed one or more isomers of monohydroxylated dibenzofurans identified by chemical structure analysis with hplc and GC-MS, indicating an active monooxygenase system widely distributed in filamentous fungi. Dihydroxylated derivatives were detected in only very low amounts and some degradation products were observed. A few fungi metabolized dibenzofuran only to intermediates with hydrophilic characteristics (eight strains) and nine strains showed no transformation capability. The transformation reactions were not inducible and seemed to be strain-specific.