A periodontal pathogen, Porphyromonas gingivalis possesses either a 53 kD (Ag53) or a 67 kD (Ag67) outer membrane protein (OMP). Almost all sera from patients with periodontal diseases reacted strongly with either Ag53 or Ag67. In previous work the cloning and sequencing of the 53 kD outer membrane protein gene designated pga53 from P. gingivalis FDC381, was reported and the presence of a gene homologous to pga53 in P. gingivalis ATCC 33277 demonstrated. In the present work this pga53-homologous gene from P. gingivalis ATCC 33277 was isolated and characterized. Nucleotide sequence analysis revealed that this gene encoded Ag67, and the gene was designated pga67. The deduced amino acid sequence and composition of pga67 was similar to the amino acid composition and N-terminal partial sequence of Ag67. An open reading frame of pga67 consisted of 1,692 nucleotides encoded as 564 amino acids, including a 49 amino acid signal sequence. The comparative analysis between pga67 and pga53 revealed that (1) the deduced amino acid sequence showed a 30.1% homology; (2) signal sequence and proline-rich regions at the C-terminus were the most conserved regions; (3) considerable differences were found mainly in the middle part of the OMPs; and (4) obvious differences in the two-dimensional models were evoked. These differences between pga67 and pga53 may explain the antigenic diversity between Ag67 and Ag53 OMPs.