The rglA and rglB genes code for two different proteins which cleave the hydroxymethylated cytosine residues of T-even phages. We isolated Tn10 and Tn5 insertion mutants of the above genes and of the genes in and around the rglA and rglB loci. These insertions were used to construct a detailed genetic map. Our results show that the rglA gene maps at 25.24 min and the rglB gene at 98.39 min on the standard Escherichia coli K12 genetic map.