Microcalorimetry, i.e., the measurement of heat production rate was used to assess the metabolic activity of six wood-decay fungi growing on moist wood particles as a function of the temperature. Four brown-rot fungi increased their activity with increasing temperature in the temperature range 5-35 C, and one brown-rot and the white-rot fungus had a lower activity at 35 C than at 25 C. It may be possible to estimate the relative growth rate of a fungus in relation to different environmental factors by measurements of the production of heat if there are no transient effects. The possible use of the results is discussed for three applications: hazard evaluation of large piles of moist biological fuels, brown-rot degradation of wood constructions, and detoxification by white rot fungi of soils containing recalcitrant toxic chemicals