Selection of a large number of different strains of hyphal fungi of the genusAspergillus, capable of production of extracellular mannosidase and mannanase type enzymes, was carried out. Before cultivating the strains on liquid synthetic medium containing 0.5%Saccharomyces cerevisiae mannan as the carbon source, they were adapted by multiple passage on solid synthetic media containingd-mannose,d-mannose and α-mannan and lastly only α-mannan. The extracellular enzymatic preparations of the mould fungi were tested for their ability to hydrolyse three different substrates—Saccharomyces cerevisiae, Torulopsis ingeniosa andTorulopsis colliculosa mannan. The production of α-mannosidase was found to be specifically dependent on the character of the substrate used for cultivation of the fungus.