The 5’-3’ exonuclease of bacteriophage T5 shows amino acid sequence homology to E. coli DNA polymerase I. The homologous regions of Pol I are located in the amino-terminal domain (small proteolysis fragment), which contains 5’-3’ exonuclease activity of that enzyme. The similarity of amino acid sequence and catalytic function of these two proteins implies they are evolutionarily related and that T5 exonuclease can be included in a small family of proteins which share sequence similarity with Pol I.