Acinetobacter species are defined on the basis of several phenotypic characters, results of DNA-DNA homology, and more recently, similarities or dissimilarities in 16S rRNA gene sequences. However, the 16S rRNA gene is not polymorphic enough to clearly distinguish all Acinetobacter species. We used an RNA polymerase -subunit gene (rpoB)-based identification scheme for the delineation of species within the genus Acinetobacter, and towards that end, we determined the complete rpoB gene and flanking spacer (rplL-rpoB and rpoB-rpoC) sequences of the 17 reference strains of Acinetobacter species and 7 unnamed genomospecies. By using complete gene sequences (4,089 bp), we clearly separated all species and grouped them into different clusters. A phylogenetic tree constructed using these sequences was supported by bootstrap values higher than those obtained with 16S rRNA or the gyrB or recA gene. Four pairs of primers enabled us to amplify and sequence two highly polymorphic partial sequences (350 and 450 bp) of the rpoB gene. These and flanking spacers were designed and tested for rapid identification of the 17 reference strains of Acinetobacter species and 7 unnamed genomospecies. Each of these four variable sequences enabled us to delineate most species. Sequences of at least two polymorphic sequences should be used to distinguish Acinetobacter grimontii, Acinetobacter junii, Acinetobacter baylyi, and genomic species 9 from one another. Finally, 21 clinical isolates of Acinetobacter baumannii were tested for intraspecies relationships and assigned correctly to the same species by comparing the partial sequences of the rpoB gene and its flanking spacers.Acinetobacter species are ubiquitous in the environment and have emerged as important nosocomial pathogens (3). A propensity to tolerate drying (29, 30) and resistance to many commonly used antibiotics (28) are key factors in enabling the organism to survive and spread in the nosocomial environment. Furthermore, Acinetobacter species may serve as reservoirs of antibiotic-resistant genes, particularly in hospital environments (22).The genus Acinetobacter, originally proposed by Brisou and Prevot (6), comprises a collection of bacteria which show a great deal of phenotypic similarities. Currently, the genus Acinetobacter comprises many taxons based on hybridization groups and classified as genomovars and genomospecies. From an official taxonomic point of view, there are currently 17 Acinetobacter nomenspecies with standing in nomenclature. However, several other genomospecies have been proposed, mainly on the basis of DNA hybridization studies (7,20,21,28).Delineation of species within the genus Acinetobacter is often problematic (22). A number of recent studies have attempted to set up reliable identification schemes based on DNA homology, phenotypic characters, and comparison of cell envelope and outer membrane protein patterns, but such methods yield inconsistent results (1,10,11,12,23). The phenotypic characters of Acinetobacter species are susceptible to the condit...