Persistent lethal sectoring in a homothallic strain of yeast has been ascribed to tetrasomy for chromosome I. Such aneuploids can appear many generations after irradiation. The data thus indicate that an induced predisposition towards aneuploidy can be prolonged through successive post-irradiation cell divisions. Sporadic cell death in tetrasomics for chromosome I was found to result from a metabolic imbalance and not from a genetic instability conseqent to aneuploidy. This imbalance may be due to a dosage effect involving cistrons for ribosomal RNA since many of these are known to be located on chromosome I. Tetrasomy is not the only cause of persistent lethal sectoring; the phenomenon has been initiated through genetic recombination involving normal diploids. It has also been concluded that, in trisomics, equational division of the supernumerary chromosome sometimes occurs at the first meiotic division.