During the isolation of RNA and DNA ligase from extracts of phage T4 infected Escherichia coli [ 1,2], I observed phosphatase activity which attacked 5'-phosphates of the oligonucleotide substrates. Since a characterized phosphatase would be useful in the structural studies of nucieotides and nucleic acids, puri~cation of the phosphatase activity was undertaken. The phosphatase activity is also present in uninfected Escherichia coli Al 9 extracts and the enzyme has unique substrate specificity.