A host vector system in Gluconobacter oxydans was constructed. An Acetobacter-Escherichia coli shuttle vector was introduced with the efficiency of 10(4) transformants/microg of DNA. Next, aiming for a self-cloning vector, we found a cryptic plasmid (which we named pAG5) of 5648 bp in G. oxydans strain IFO 3171, and sequenced the nucleotides. The plasmid seemed to have only one open reading flame (ORF) for a possible replication protein. Shuttle vectors of Gluconobacter-E. coli were constructed with the plasmid pAG5 and an E. coli vector, pUC18.