The UV induced mutation frequency of a given base pair located at different sites within the CYC1 gene of Saccharomyces cerevisiae was found to vary by more than fifty fold, indicating the existence of hotspots and coldspots typical of those found in other organisms. We were unable, however, to find any feature of the nucleotide sequence at or near the sites of mutation that explains this variability. These and other data suggest that hotspots are not located within regions particularly susceptible to the formation of premutational lesions. More probably the variation in mutability depends on differences in the activity of enzymes responsible for producing mutations, though the reasons for these differences are not understood and may depend on factors not directly related to nucleotide sequence.