Four monosporous cultures of Cyathus stercoreus were tested and assigned to 4 mating types: AB, ab, Ab, and aB. In pairings of compatible mycelia, both mycelia usually became diploid. Unilateral diploidization was constantly associated with the binucleate condition of the acceptor mycelium. Pairings of incompatible mycelia with B or b factors in common gave a line of demarcation in the region of contact, with no evidence of nuclear migration. Incompatible pairings with A or a factors in common gave no demarcation line, and the resulting mycelium was "blotchy" and heterocaryotic. Pairings of uninucleate AB mycelia produced a binucleate mycelium and pairings between a uninucleate and a binucleate AB mycelium gave a mixed mycelium. In each case the resulting mycelium showed unilateral diploidization with ab mycelium. The factors B-b seem to control nuclear migration, while A-a are associated with the dicaryotic condition.