Mutations induced by UVB radiation and natural sunlight in a plasmid-borne yeast (Saccharomyces cerevisiae) tRNA gene (SUP4-o) were characterised by DNA sequencing. For both agents, the majority (> 90%) of the total mutations analysed were single base-pair substitutions, but tandem substitutions and single base-pair deletions also were detected. Each agent induced all six types of base-pair change but the tandem substitutions involved exclusively G.C-->A.T transitions. However, the fractions of single and tandem G.C-->A.T transitions were reduced by about 50%, and the fraction of transversions at G.C pairs was increased by 11-fold for sunlight relative to UVB. Comparisons of the site and strand specificities of the substitutions suggested that dipyrimidine adducts were responsible for the transitions, and that other lesions induced by sunlight may have given rise to the transversions. The relevance of these findings to skin cancer is discussed.