Bacillus anthracis is a large, gram-positive, aerobic, spore-forming bacillus. Its endospores do not divide, have no measurable metabolism, and are resistant to drying, heat, UV light, gamma radiation, and many disinfectants. In some cases, spores can remain dormant for decades. B. anthracis causes a zoonotic disease, anthrax. It also causes acute and often lethal disease in humans, such as cutaneous, intestinal, and pulmonary anthrax. For a long time, this species has attracted attention because of its hardiness, dormancy, and thus its potential use as a biological weapon (12, 13). In October 2001, B. anthracis spores were used to attack human populations in Florida, New Jersey, New York, and Washington, D.C. (12), which heightened public awareness and concern about anthrax. B. anthracis infections are confirmed mainly by conventional microbiological methods, i.e., Gram staining, capsule staining, colony morphology, and biochemical characteristics (4, 18). However, because of its clinical importance and its implication concerning public security, suspected specimens are usually referred to public health laboratories for definitive identification, epidemiologic study, and susceptibility testing (28). Therefore, not only precise but also rapid identification of isolated Bacillus species is needed. In addition, it is also important to know whether detected or isolated B. anthracis strains contain virulence plasmids or not because the virulence of B. anthracis is related to encapsulating and toxin-encoding plasmids. Given this situation, genotype analysis would seem to be most appropriate for the precise differential identification of virulent B. anthracis. However, genotype analysis is not straightforward for several reasons. Phylogenetically, B. anthracis is considered a member of the “B. cereus group,” which also includes B. cereus, B. thuringiensis, and B. mycoides (18). Moreover, B. anthracis is genotypically differentiated from its close relatives, B. cereus and B. thuringiensis, only by the presence of toxin-encoding plasmids (19), and the genomes of these three species show high levels of similarity. For example, this group share almost identical 16S ribosomal DNA sequences (1), and for this reason were suggested to be one species based on multilocus enzyme electrophoresis (MLEE) (11). Moreover, the genome of B. anthracis has 11 rRNA operons, which show sequence polymorphisms at 10 positions (27). Analysis of other chromosomal genes such as gyrB (9, 35) and the 16S-23S ribosomal intergenic spacer (2), which are usually used for bacterial genotyping or phylogenetic analysis also failed to discriminate B. anthracis from B. cereus and B. thuringiensis. Furthermore, it seems to be even more difficult to differentiate them by plasmid gene analysis, because of plasmid transfer among the closest species. For example, genes in the plasmid of B. anthracis have been successfully expressed in other bacteria (30) and been reported in other Bacillus species (22). It is important to note that pXO2 can be lost naturally (32). Due to the natural competence of B. thuringiensis and B. cereus, the horizontal transfer of plasmids has been reported (8, 26, 35). The findings presented above show why the detection and identification of B. anthracis from clinical or environmental samples must be performed precisely and why B. anthracis-specific chromosomal markers should be developed to differentiate B. anthracis from its closest relatives (23). The rpoB gene, encoding the RNA polymerase β-subunit, has been used as a marker for bacterial identification and for phylogenetic study (5, 6, 14, 16, 17, 20, 25). Recently, the rpoB gene was used for the real-time PCR detection of B. anthracis (23); however, false-positive results were observed. According to Ellerbrok et al. (7), B. cereus and B. megaterium strains were also detected by real-time rpoB PCR and, therefore, a more reliable detection and identification method is required for B. anthracis chromosomal DNA. In the present study, partial rpoB sequences (318 bp), which are located downstream of those used for real-time PCR (23) and which contain a region related to rifampin resistance, Rif r (21, 33), were compared for the genotyping of B. anthracis, B. cereus, B. thuringiensis, B. mycoides, and B. megaterium. Subsequently, we undertook to identify five Korean isolates based on their rpoB sequences and to develop a simple multiplex PCR method that can be used for the rapid and differential detection of virulent B. anthracis.