THE 5’-exonucleases are enzymes that are essential for DNA replication and repair(1). As well as their exonucleolytic action, removing nucleotides from the 5’-end of nucleic acid molecules such as Okazaki fragments(2), many 5’-3’-exonucleases have been Shown to possess endonucleolytic activities(3,4), T5 5’-3’-exonuclease shares many similarities with the amino termini of eubacterial DNA polymerases(5), although, unlike eubacteria, phages such as T5, T4 and T7 express polymerase and 5’-exonuclease proteins from separate genes, Here we report the 2.5-Angstrom crystal structure of the phage T5 5’-exonuclease, which reveals a helical arch for binding DNA, We propose a model consistent with a threading mechanism in which single-stranded DNA could slide through the arch, which is formed by two helices, one containing positively charged, and the other hydrophobic, residues. The active site is at the base of the arch, and contains two metal-binding sites.