A pair of primers were designed based on the DNA sequence of the outer membrane protein A gene (ompA)in Genbank. With the special primers, ompA of E.coli O_2, O_(78) and their fusion bivalent attenunated strain O_(2,78) (Chl~r Nor~r) were amplified by PCR. After sequencing , all of the three ompA were shown to contain (2 276) nt. Starting at nucleotide (1 037) and ending at nucleotide (2 077), an open reading frame(ORF) of (1 041) bp encodes the pro-OmpA protein consisting of 346 amino acid residues. Hydrophobicity analysis suggested that the preceeding 21 amino acid constitute a typical signal sequence, and that the mature form of OmpA is 325 amino acid long with a relative molecular mass of (37 000). Analysis of the nucleotide sequences of the three ompA genes of avian Escherichia coli showed that they were all identical. These results indicated that O_2 and O_(78) have the same outer membrane protein A antigen.