Abstract: In the laboratory, decaying wood produces measurable quantities of carbon monoxide as measured with on infra-red gas analyzer. The highest CO ooncentration 205 ppm, was obtained from birch wood, colonized by Piptoporus betulinus. Pure cultures of wood-decomposing fungi grown on malt agar give a positive reaction with palladium chloride. The tolerance of 32 wood-decomposing species of Hymenomycetes was determined by growing them in desiccators, into which measured amounts of carbon monoxide were added. All species proved to grow well in 1:1 CO – air mixtures and most of them grew in 3:1 mixtures, suggesting that in the ecology of wood-decomposing fungi CO is of minor importance.