ABSTRACT When ascospores from crosses of certain strains were germinated under conditions selective for heterozygosity of complementing markers on one linkage group, a portion of the resulting colonies were also heterozygous for unselected markers on other chromosomes, implying multiple disomy. The frequency of disomy and the pattern of marker homozygosity are consistent with most or all multiple disomics having originated as complete diploids following nondisjunction at meiosis I. The production of diploid ascospores in these strains is apparently under polygenic control. The diploids are highly unstable and do not differ from n+1 disomics in rates and mechanisms of haploidization and mitotic crossing over.