SINGLE-STRANDED DNA can fold into well-defined sequence-dependent tertiary structures that specifically bind a variety of target molecules(1-10), raising the possibility that some folded single-stranded DNAs might exhibit catalytic activities similar to those of ribozymes and protein enzymes. Derivatives of the hammerhead ribozyme that contain a majority of deoxyribonucleotides retain the ability to cleave RNA(11), and a ’deoxyribozyme’ was generated by leaving all essential ribonucleotides of the hammerhead on the RNA ’substrate’(12). Recently in vitro selection has been used to isolate a DNA sequence that shows Pb2+-dependent RNA-cleaving activity(13). Here we report the isolation by in vitro selection(14-17) of a small single-stranded DNA that is a Zn2+/Cu2+-dependent metalloenzyme. The enzyme catalyses the formation of a new phosphodiester bond by the condensation of the 5’-hydroxyl of one oligodeoxynucleotide and a 3’-phosphorimidazolide on another oligodeoxynucleotide, and shows multiple turnover ligation.