The amount and quality of DNA synthesized in excision-defective strains of yeast was assayed using alkaline sucrose gradients. It was observed that, in such strains, there was less newly synthesized DNA in irradiated cells and that this material was in smaller pieces than in unirradiated controls. The molecular weight was inversely proportional to dose. The low molecular weight DNA chased into normally high molecular weight material during a post-pulse incubation period. This chase was inhibited by higher doses of UV, by hydroxyurea, and required the function of the RAD18 gene. The RAD6 gene function was necessary to prevent degradation of template DNA in irradiated excision-defective strains.