NATURALLY occurring genetic variation has been demonstrated in several species belonging to the Hymenomycetes, as well as in other fungi. The best known examples are of course the incompatibility factors found in the heterothallic species ( WHITEHOUSE 1949). Other major genes segregating among progenies of wild dikaryons were found to affect the shape of the fruit bodies (ZATTLER 1924; RAPER and KRONGELB 1958; JURGENS 1958; KIMURA and FUJIO 1961 ) and the morphology of monokaryotic mycelia (CROFT and SIMCHEN 1965). As in higher plants and animals, most heritable wild variation in fungi, however, is continuous and highly sensitive to environmental conditions; it is therefore more difficult to detect than major gene differences. Such heritable variation has been shown to exist for growth rate of monokaryons in Collybia uelutipes (CROFT and SIMCHEN 1965) and in Schizophyllum commune (SIMCHEN 1966). Demonstration of heritable variation for dikaryotic traits requires the mating of monokaryotic lines, and is therefore not as simple as the demonstration of monokaryotic variation. KNIEP (1928) has already suggested that the ability of the Hymenomycetes to fruit is determined by the genotype, although not necessarily by major genes. GILMORE (1926), BRODIE (1948), BARNETT and LILLY (1949) and RAPER and KRONGELB (1958) have in fact shown experimentally that fruiting ability is controlled by the genotype in a way that cannot be explained by simple major gene models. Investigations of continuously varying characteristics require suitable genetical tools, which differ from those used to investigate single gene differences. Suitable biometrical methods have been developed in our laboratory, and used to investigate the genetic control of dikaryotic growth rate among progenies of two wild isolates of S. commune ( SIMCHEN and JINKS 1946). The same methods are used here to obtain further information about the determination of growth rate of dikaryons in this fungus, as well as to investigate the genetical control of other characters, such as fruiting time and fruit weight.