1. The numbers and distribution of mating-type factors in local populations of Polystictus versicolor have been studied, and the data available on the number and distribution of mating-type alleles in other fungi have been reviewed. Possible explanations for the low numbers recorded in Polyfitictus versioolor are -given, and the potential out-breeding capacity of the local populations is discussed. 2. Growth rates of dicaryotic and monocaryotic strains of P. versicolor at 25°C and 35°C. have been studied. 3. Studies have been made on the genetical and physiological nature of the mycelium in P. versicolor and it is thought that the mycelium exists in nature as a physiologically homogenous and genetically heterogenous unit. The mechanisms by which such a mycelium could be established have been studied. They are (a) hyphal fusions and (b) dj. x mon. pairings. 4. The effects of different mating-type factors in compatible pairings on (a) hyphal fusion; (b) nuclear migrations; (c) clamp connection formations have been investigated, and a suggestion made concerning a model for gene action in connection with the formation of clamps.