We have previously cloned the gene encoding the cell division protein FtsZ, designated PgFtsZ, from Porphyromonas gingivalis, an oral anaerobic bacterium implicated in advanced periodontal disease. In the present study, we have shown that overexpression of ZDeltaC02, a mutant form of PgFtsZ in which 128 amino acid residues have been removed from the C-terminus, caused an inhibition of cell division in E. coli cells. However, overexpression of ZDeltaC03, missing 177 residues from the C-terminus, did not inhibit cell division, suggesting that the 49 residues between 281 and 329 are required for cell division. Sequence comparison of the known prokaryotic FtsZs revealed that this region contained a highly conserved domain, designated A-domain, in which Ala320 of PgFtsZ was conserved throughout a broad variety of species, Therefore, we analyzed the role of Ala320 by site-directed mutagenesis. We found that overexpression of ZA320H and ZA320R resulted in the normal phenotype, unlike the wild type. Immunoblot analysis showed that these mutant proteins were expressed at similar levels. These results suggest that Ala320 is highly conserved and is crucial for cell division.