Action spectra for the induction of intragenic mitotic recombination (gene conversion) at the trp 5 locus by UV are presented for three cell stages (T0, T9 and T16) taken from synchronously growing cultures of Saccharomyces cerevisiae. The spectra over the range from 230 to 300 nm were taken mostly in 5-nm steps. The peak of action spectra was significantly shifted, regardless of the stage, toward the longer wavelengths as compared with that of the absorption spectrum of DNA (258 nm) or even that of thymine (265 nm). In one extreme case (T16), the peak was shifted 17 nm from the absorption peak of DNA. Further, the spectrum changed its shape as the cell stage advanced from non-dividing (unbudded) (T0) to a dividing phase (T16). Furthermore, the induction cross section decreased by a large factor (about 40), regardless of the wavelength, in going from T0 to T16. From observations of the high photoreversibility of induced conversions, the major primary damage was thought to be pyrimidine dimers in the DNA. One plausible explanation, though not quite satisfactory from the quantitative viewpoint for these findings was that the increasing RNA during growth would screen the incident UV differentially with respect to the stage. If this explanation is correct, thymine dimers may still be considered, in spite of the shifts and deformations in the action spectra, as the major primary damage that triggers the long series of processes leading to gene conversion. Conventional methods for obtaining action spectra are discussed in comparison with the present method, which was based on sensitivity parameter a in the proposed dose (t)-frequency (f) relation, f = (at)alpha (alpha is the multiplicity parameter).