The thermophilic fungus Thermomyces lanuginosus, which is able to use dextran as primary carbon source for growth, excreted during the early phases of growth an enzyme activity capable of degrading dextran. The activity peaked at 22 h and decreased rapidly after the culture entered the stationary phase, probably caused by protease activity, Results from growth on a number of different carbon sources showed that polymer carbohydrates yielded the highest dextranase activities. On the basis of the substrate specificity and the release of glucose in the alpha-anomeric form from the hydrolysis of maltose, it is proposed that the enzyme responsible for the necessary degradation of dextran to smaller saccharides is an alpha-glucosidase.