Abstract: The mating system of Phlebiopsis gigantea (Fr.) Jülich, a common decay fungus of coniferous wood and a biological control agent used against Heterobasidion annosum (Fr.) Bref., is shown to be bipolar. Clamp connections can be found in both hetero- and homokaryotic mycelia. Homokaryotic fruiting occurs fairly frequently in pure cultures. Collections from Finland, Poland and France proved to be interfertile.