Heterokaryons capable of segregating clones of different phenotypes were obtained in saccharomycetes yeast. Clones expressing phenotypic traits encoded by the nuclear genome of one parent and the mitochondrial genome of another were shown to contain cells that carried the second phenotypically silent nucleus. The cryptic nucleus can be uncovered by growing these XcytoductantsX on the corresponding medium. The use of strains carrying an insertion in the URA3 gene and the endogenic plasmid, 2 microns DNA, made it possible to detect this nucleus by blot hybridization.