UV induced adenine-requiring mutation in yeast cells was investigated quantatively at several wavelengths of monochromatic UV and at maximum photoreversion conditions, using a diploid strain of Saccharomyces cerevisiae heterozygous for adenine locus. Mutant production by UV and the remaining mutant, after subsequent treatment, by visible light at maximum photoreversion, both increased almost linearly against the UV dose, irrespective of the wavelengths used except for some deviation in the shorter wavelengths. Using a scattering factor of UV at the cell boundary, reliable absolute cross sections of the mutation were obtained for several wavelengths. Some implications for genetics of the findings were discussed with special references to the relationship between mutation production and the inducible photochemical changes in DNA bases by UV.