The stimulation of the germination of Agaricus bisporus spores by mycelium of the same species has been shown to be due to a volatile metabolite, diffusing into the culture medium and into the atmosphere. A wide range of other fungi has been found to affect A. bisporus spores in a similar manner. There was no evidence . that the stimulant was carbon dioxide. A seven-carbon olefin, isolated from air, which had been passed through cultures of A. bisporus mycelium and dried with phosphorus pentoxide, showed germination-stimulating activity but may have been produced by the action of this drying agent on some other metabolite from the mycelium. Volatile materials reported by other workers from A. bisporus mycelium and from Saccharomyces cereviseae were tested and, of these, iso-valeric acid and iso-amyl alcohol were found to stimulate spore germination of A. bisporus