Abstract
In artificial culture, Aureobasidium pullulans (de Bary) Arnaud produced a stimulatory effect on rhizomorph development of Armillaria mellea (Fr.) Quél. Tests were carried out on malt agar, potato dextrose agar, and a chemically defined synthetic medium. The stimulatory substance (or substances) produced by A. pullulans was extracellular, diffusible in the medium and volatile. Tests indicated that the stimulation was caused by something other than ethanol or indoleacetic acid.