Porphyromonas gingivalis is a gram-negative anaerobic bacterial species strongly associated with adult periodontitis. One of its distinguishing characteristics and putative virulence properties is the ability to agglutinate erythrocytes. We have previously reported the cloning of multiple hemagglutinin genes from P. gingivalis 381. Subsequent sequencing of clone ST 2 revealed that the cloned fragment contained only an internal portion of the gene which lacked both start and stop codons. We here report the cloning and sequencing of the entire gene, designated hagA, as well as its relationship to other genes of this species. By use of inverse PCR technology and the construction of several additional genomic libraries, the complete open reading frame of hagA was found to be 7,887 bp in length, encoding a protein of 2,628 amino acids with a molecular mass of 283.3 kDa, which is among the largest genes ever cloned from a prokaryote to date. Within its open reading frame, four large, contiguous, direct repeats (varying from 1,318 to 1,368 bp) were identified. The repeat unit (HArep), which is assumed to contain the hemagglutinin domain, is also present in other recently reported protease and hemagglutinin genes in P. gingivalis. Thus, we propose that hagA and the other genes which share the HArep sequence form a multigene family with hagA as a central member.