Cell-free extracts of the unicellular algae Chlorella pyrenoidosa induce a UV-dependent incision of both phi x 174 RFI DNA and a 27-mer double-stranded oligonucleotide containing a single thymine dimer, as analyzed by agarose and denaturing polyacrylamide gel electrophoresis, respectively. The incision reaction occurred only on the thymine dimer-containing strand of the 27-mer, and a major incision product of 17-mer was observed. Addition of ATP or ATP-regenerating system to algal extracts increased the effect of DNA incision.