Recently, several species of basidiomycetes were shown to produce de novo high concentrations of chloroaromatic metabolites. Since these lignocellulose-degrading fungi play a major role in the ecosphere, the purpose of this study was to determine the ubiquity of organohalogen production among basidiomycetes. A total of 191 fungal strains were monitored for adsorbable organic halogen (AOX) production when grown on de lined Liquid media. Approximately 50% of the strains tested and 55% of the genera tested produced AOX. A low production of 0.1-0.5 mg AOX/l was observed among 25% of the strains, a moderate production of 0.5-5.0 mg AOX/l was observed among 16% of the strains and 9% of the strains produced high levels (5-67 mg AOX/l). The latter group was dominated by species belonging to the genera Hypholoma, Mycena and Bjerkandera, showing specific AOX productions in the range 1074-30893 mg AOX/kg dry weight of mycelial biomass. Many highly ecologically significant fungal species were identified among the moderate to high producers, These species were also able to produce AOX when cultivated on natural lignocellulosic substrates. NL Hypholoma fasciculare and Mycena metata respectively produced up to 132 mg and 193 mg AOX/kg dry weight of forest litter substrate in 4 weeks.