A genetically stable nontoxinogenic form of the Ogawa 395 strain of Vibrio cholerae which has a deletion mutation in both copies of the A.sub.1 -peptide-encoding gene, resulting in the loss of a gene sequence required for expression of a toxic A.sub.1 peptide is disclosed, as well as plasmids and methods for making the strain. The strain is useful as a live or dead oral vaccine.