In the wood destroying fungus Schizophyllum commune, a well known subject for genetic studies, fruit bodies are produced not only in the course of the sexual cycle but also asexually. Sexual fruiting requires the establishment of a dikaryon which is under the control of the incompatibility factors A and B. Asexual fruiting, however, starts directly from a monokaryon. The initiation of monokaryotic fruiting requires the presence of a single gene leading to the differentiation of fruit body initials. The action of at least two more genes is required for the further morphogenetic steps resulting eventually in the production of fruit bodies which differ in shape from dikaryotic fruit bodies. As a consequence of a mitosis their basidia produce two spores only. The three genes responsible for monokaryotic fruiting are pleiotropic and determine synergistically the timing of dikaryotic fruiting within a range between 6 to 20 d or longer. A fourth gene was found which codes epistatically for the formation of dome-like masses of stromatic tissue, thus directing morphogenesis into a side track.