Nitrosospira, Nitrosolobus, and Nitrosovibrio, three genera of ammonia-oxidizing bacteria (AOB), are morphologically distinct from each other (Nitrosospira: spiral, Nitrosolobus: lobate, Nitrosovibrio: slender curved rods). However, these three genera cannot be classified using the phylogenetic tree based on 16S rDNA sequences. Phosphoglycerate kinase (PGK) was purified from 10 AOB strains as an index for identification, and its enzymatic properties were investigated. No distinction could be made between these three genera of AOB, because there was no notable difference in the molecular weight, K-m, or other properties of their PGKs. However, the N-terminal amino acid residues of PGK of Pro(P)-1, Ile(I)-6 and Asp(D)-12 have sequences characteristic of genus Nitrosovibrio (three strains) in four AOB genera. Therefore, it is proved possible to use the differences in the sequence to identify individual strains of the genus Nitrosovibrio.