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SUMMARY: Heterokaryons obtained between nutritional mutants of the imperfect fungus Verticillium albo-atrum (Reinke & Berth) were very unstable, and heterokaryotic hyphal tips growing out from mixed inocula were difficult to find. Heterokaryotic conidia were not detected, in spite of the fact that about 1% of the spores contained two or three nuclei. Heterozygous nuclei in uninucleate conidia from mixed cultures yielded recombinant pheno-types at a higher frequency than usually observed in the other imperfect fungi which have been studied genetically. With no easily applicable criterion of chromosome number, such as spore size, the interpretation of results is difficult, but the genetic recombination demonstrated here seems likely to be accomplished through a parasexual system. Nuclei heterozygous at some loci but homozygous at others were formed from nuclei heterozygous at four marked loci, and were detected by isolating second-order segregants. These heterozygous segregants were presumably produced by mitotic recombination.