AbstractThe wavelength dependence of inactivation and mutagenesis in the range between 254 and 313 nm was investigated in haploid cells of the yeastSaccharomyces cerevisiae.The action spectra for a wild type and an excision deficient mutant essentially corresponded to those for bacteria and mammalian cells. There were, however, minor differences. The ratio of mutagenic to lethal action decreased with increasing wavelength in the wild type, but remained constant in the excision deficient mutant. This is discussed in terms of error‐prone and erro‐free repair. For both inactivation and mutagensis, photorcactivability was lower with longer wavelengths in the wild type, but did not change in the excision deficient mutant. Implications for the role of pyrimidine dimers are conside