Both Armillaria mellea and Stereum hirsutum produced pseudosclerotial plates (PSP) under conditions of maximum humidity. Moisture loss was therefore excluded as a possible stimulating factor in plate formation. Controlled drying experiments, using laboratory-infected beech samples, showed that with A. mellea a relationship existed between the moisture content of the substrate during the initial period of PSP formation and the ability of the fungus to form a PSP. Inhibition always occurred at the preliminary stage of hyphal proliferation. In the case of S. hirsutum only segments exposed to r.h. 100% formed PSP. Results indicated that the more delayed hyphal proliferation stage characteristic of S. hirsutum may be a critical factor determining the greater susceptibility to moisture stress of PSP formation in this fungus.