The fate of ultraviolet-induced, thymine-containing dimers in the deoxyribonucleic acid (DNA) of Bacillus subtilis was investigated in both the wild type (UVR) and an ultraviolet light-sensitive (UVS) mutant. During incubation in the dark, dimers were excised from the DNA of the UVRB. subtilis, but remained in the DNA of the UVS mutant. About 40% of the excised dimers recovered in the wild type were in the acid-soluble fraction; the remainder were in the incubation medium. A UVS mutant of Escherichia coli K-12, shown previously to be defective in dimer excision, was irradiated with ultraviolet light and incubated under visible light for 3 hr. About 65% of thymine-containing photoproducts were removed from the DNA. These photoproducts were not recovered in the acid-soluble fraction. In comparison, the UVS mutant of B. subtilis lost only 13% of such photoproducts from DNA when exposed to light under the same conditions.