The production of chloromethane (CH3Cl) by wood rotting fungi of the Hymenochaetaceae is discussed with particular emphasis on emissions by species of Phellinus and Inonotus. Recent work on the metabolic role of CH3Cl as a methyl donor in the biosynthesis of secondary metabolites both in the Hymenochaetaceae and other families of white-rot fungi is reviewed. The parameters affecting the fungal emissions of CH3Cl in forest ecosystems are considered and where possible quantified. The annual global input to the atmosphere from this source is provisionally estimated at 160000 t of which 75% is released from tropical and subtropical forests and 86% is attributable to Phellinus. The possible impact of the contribution from fungi and other biological sources on the atmospheric CH3Cl burden and stratospheric ozone depletion is assessed.