SUMMARYA striking variant has been produced by cultivation of monoploid hyphae ofHypomyces Ipomoeaeon medium containing a sub-lethal concentration of zinc nitrate.Analyses of 5 consecutive single-conidium culture-series, consisting of 25 cultures each, have proved the mutant strain,aborta, to be homocaryotic. Single-conidium and mass-transfer cultures of theabortastrain have shown a high degree of uniformity.The most striking feature associated with the variant was that four, rather than eight, spores were formed inaborta×normalasci, due apparently to inability of nuclei carrying theabortafactors to delimit spores.Evidence strongly indicated that in rare instances ascospores inaborta×normalasci may include more than one of the eight nuclei resulting from maturation of the zygote nucleus. Thus, in two instances, single ascospores isolated from hybrid perithecia yielded mixed cultures containing bothnormalandabortahyphae. The two fxabortastrains thus obtained bore opposite sexual-reaction factors. In two other instances, single ascospores from hybrid perithecia yielded cultures containingnormalhyphae of both sexual reactions, as was evidenced by the appearance of fertile perithecia in both cultures.The inability ofabortanuclei to delimit ascospores may be attributed to the inactivation or to the deletion of one or more genes determining ascospore delimitation. The ascospore-delimitation factors and the sexual-reaction factors are in different linkage groups.The genie alteration or mutation, although having occurred on medium containing a sub-lethal concentration of zinc, cannot be considered a specific effect of zinc ion without further evidence.