Armillaria mellea(Fr.) Quél, was able to develop an extensive rhizomorph system in autoclaved soil in two-section petri plates if the stimulatory substance produced byAureobasidium pullulans(de Bary) Arnaud was present. A sufficient quantity of the substance was produced byA.pullulanson a glucose–asparagine agar substrate but not on an autoclaved soil substrate.