Abstract: The effect of carbon dioxide on the spore germination of 23 wooddecomposing Hymenomycetes was investigated by introducing 1 %, 5 %, and 20 % carbon dioxide – air mixtures into desiccators, in which spores were germinated on Hagem agar. Compared to ordinary air, low concentrations (1 % and 5 %) markedly stimulated the germination rate of several species. Species growing in living tree stems were stimulated most effectively (Spongipellis borealis, Polyporus squamosus, Pholiota aurivella and Fomitopsis annosa). Stimulation took place at different pH values and seems to be independent of the acidifying action of carbon dioxide. Ecological aspects of the stimulation are discussed.