PREVIOUS investigations on tetrapolar sexuality in Basidiomycetes1–3 have shown that morphologically aberrant sectors arise in surprisingly high frequency in the common-A heterokaryons of Schizophyllum commune. These heterokaryons are established whenever mat ings are made of two strains which share common factors at one of the two complex loci governing incompatibility, the A factor1. On further isolation these sectors are capable of growing into homokaryotic mycelia of a limited number of phenotypes, and in the majority of cases each type can be shown to be the result of a single-gene mutation4. No such occurrence of visibly expressed mutations has ever been observed within homokaryons. Among the hypotheses which have been proposed to account for the origin of these expressed mutations are3 : (a) increased mutagenesis within the heterokaryon at a few specific loci ; (b) increased mutagenesis at all loci followed by selection of the few specific types encountered ; (c) no increase in mutagenesis, but selection of a few specific types of low-level, spontaneous mutation.