Experiments were carried out to investigate genetic relationships within a population of Coriolus versicolor (a polyporaceous basidiomyceto causing white rot of wood) present in a birch stump. The population consisted of individual dikaryons occupying longitudinally continuous columns of decay separated from one another by narrow, dark, relatively undecayed interaction zones. These dikaryons were shown to be genetically homogeneous throughout their respective decay columns by dedikaryotization procedures. They were mutually antagonistic when paired in culture, but monokaryons derived from their fruit-bodies were interfertile. Experiments using synthesized dikaryons indicated that antagonism is inevitable between genetically distinct mycelia, but that the intensity of interaction diminishes with increased relatedness. Results of pairings between synthesized dikaryons and monokaryons varied according to the relatedness of the isolates. Antagonism invariably occurred when the monokaryon contained a nucleus differing from both nuclei in the dikaryon, but this did not necessarily prevent dikaryotization. Often in this situation dikaryotized sectors developed in the monokaryon visibly separated by zones of antagonism (‘tracks’). Where the monokaryon contained the same nucleus as one of the components of the dikaryon, antagonism usually occurred initially, but normally the colonies eventually fused.