Heterozygosity at a single, multiallelic locus (‘B-factor’) leads to development of a ‘bow-tie’ reaction between homokaryotic mycelia of Stereum hirsutum paired against each other on malt agar. This reaction involves formation of a band of appressed mycelium with abnormal branching, widest at its edges, which is usually asymmetric and tends to advance preferentially into one homokaryon leading to partial or complete replacement by the other. Expression of the reaction is probably dependent on continuance of access of nuclei from a donor into an acceptor mycelium and is affected both by the particular combinaton and relatedness of the strains, and by their physiological condition. First generation sibs can be ranked according to their relative ability to accept or donate nuclei, from which the reaction pattern for particular combinations can usually be predicted. With such strains the reaction develops more rapidly than with second generation (inbred) sibs, and, unlike in the latter, often culminates in expression of somatic incompatibility. Repeated subculturing tends to advance expression of somatic incompatibility relative to the bow-tie reaction.