Abstract— The repair of UV‐irradiated DNA of plasmid Yep13 was studied in the incision defective strains by measurement of cell transformation frequency. In Saccharomyces cerevisiae, radl,2,3 and 4 mutants could repair UV‐damaged plasmid DNA. In Escherichia coli, uvrA mutant was unable to repair UV‐damaged plasmid DNA; however, pretreatment of the plasmid with Micrococcus luteus endo‐nuclease increased repair. We concluded that all the mutations of yeast were probably limited only to the nuclear DNA.