Ultraviolet radiation induces respiration-deficient variants with high frequency in both haploid and tetraploid yeasts. Variant colonies arising after irradiation, when overlaid with triphenyl tetrazolium chloride agar, can be classified as (1) “whole colony” and (2) “variegated or sectored.” The heterogeneity in respiratory phenotype of the latter type of colonies is not detectable by colony morphology prior to overlaying with tetrazolium agar. The variegated types may explain some cases of “reversion” of respirationdeficient cells to wild type. Whole colony variants can be grouped into two classes (vI and vII). The vI variants are similar to spontaneously occurring variants and fail to revert to wild type, whereas the vII variants generally revert to wild type at a low frequency. The vI type is produced most frequently and presumably results from nongenic radiation damage, while the less frequently occurring vI type results from nuclear damage.