A fungus, Aspergillus fumigatus Fres., which used bark as its sole carbon and energy source, was isolated. Difficulties arose in measuring fungal growth, since the hyphae and the bark could not be separated. Measurement of the weight loss of the solid material did not quantitatively estimate fungal growth. Therefore, two methods were developed to estimate fungal mass when the carbon and energy source is particulate and contributes to the parameter used as a measure of growth. They were based on determination of nitrogen either in the solid material or in the medium. The nitrogen concentration in A. fumigatus was found to be nearly constant throughout the growth cycle and to be independent of the carbon and nitrogen concentrations in the medium but to vary with the carbon source used.Aspergillus fumigatus was grown at 37C as a submerged culture in salts medium with finely ground bark from Picea abies as sole carbon and energy source. The bark medium was heat-sterilized before inoculation with spores. The fungus utilized cellulose and hemicellulose but not lignin. Substances solubilized from the bark contributed to the growth. The yield was the same on unextracted as on water-extracted bark, although growth was delayed on the former. Growth was rapid and comparable to growth on other polymeric polysaccharides, i.e. starch. Aspergillus fumigatus degraded 32–40% of the polymeric part of the bark within 4 days and with an economic coefficient of about 50%.