A 7.1 kb Hind III- Xho I fragment of E.coli DNA which contains the structural gene for ribonuclease II ( rnb ) has been cloned in the recombinant plasmid pDK24. At least two constitutively expressed genes are encoded on the fragment as shown by maxicell analysis. On denaturing polyarcrylamide gels RNase II appears as a single 72,000 dalton species. The approximate site of transciption initiating of the rnb gene has been mapped. Although derivatives of E. coli harboring pDK24 contained 10-fold more RNase II activity that wild type strains without the plasmid, the degradation rate of mRNA was similar in all strains tested. Strains deficient in both RNase II and polynucleotide phosphorylase appear inviable.