One of the important effects of irradiation on a large population is the induction of recessive lethal mutations which can persist in the population for an indefinite period. Although these mutant genes are recessive in regard to lethality, it has been demonstrated that they can have either an adverse or a beneficial effect on viability when in the heterozygous state (1, 2). To understand the genetic changes occurring in populations exposed to radiations, it is therefore important that the behavior of large representative samples of radiation-induced heterozygous lethals be known. In Drosophila populations, the proportion of cells containing such lethals is reduced in the first few generations after irradiation (3). A large proportion of this decline is probably attributable to gametic elimination through association of the lethals with gross chromosomal rearrangements and consequent partial sterility, rather than to selection against the heterozygotes. However, the material does not permit a discrimination, and the extent to which the heterozygotes are also at a phenotypic disadvantage is not known. It seemed possible that the use of yeast might be advantageous in a study of the phenotypes of heterozygotes for induced recessive lethals. With this organism heterozygotes can be tested in mixed populations over lengthy periods of purely vegetative reproduction in which gametic selection would, of course, be absent. Yet the proportion of lethal-bearing cells can be determined at any time by the extraction of samples, sporulation, and ascus dissection. In this paper, the adequacy of diploid yeast for such studies was investigated in an experiment designed to measure the effect of radiation-induced recessive lethals on the ability of heterozygotes to compete with irradiated lethal-free cells.