The growth-promoting effect of the thermophilic fungus Scytalidium thermophilum in mushroom compost on the mycelium of the edible mushroom Agaricus bisporus was investigated. Results obtained by others were confirmed by showing that S. thermophilum leads to an increased hyphal extension rate of the mushroom mycelium. However, it was demonstrated that hyphal extension rates were not clearly related to mushroom biomass increase rates. A number of experiments pointed strongly towards CO2 as the determinant of hyphal extension rates. In compost, CO2 is produced mainly by thermophilic fungi. Several experiments did not reveal any other specific compound produced by S. thermophilum that increases the hyphal extension rate of the mushroom mycelium.