The thn (thin) mutant strain of Schizophyllum commune, which normally lacks aerial mycelium, produced excessive aerial hyphae when surrounded by multiple wild-type (wt) colonies. When thn and wt mycelia were grown together, the radial growth rate of the thn strain was increased, whereas the wt strain was clearly expressed in its growth, resulting in complete overgrowth of wt by thn hyphae. Overgrowing even occurred when mixtures of fragmented hyphae of both strains with a hundred times excess of wt hyphae over thn hyphae were grown from a single inoculum. Growth stimulation was also observed when thn and wt colonies were separated by dialysis membrane (cut-off 6–8 kDa) suggesting that low molecular weight molecules may be involved. The observed phenomena may explain the apparently very high frequency of occurrence of the recessive thn mutation in S. commune.