Plasmid genes that are responsible for virulence of Bacillus anthracis are important targets for the DNA-based detection of anthrax. We evaluated the distribution of the Ba813 chromosomal DNA sequence (Ba813) within closely related Bacillus species. Ba813 was systematically identified from 47 strains or isolates of B. anthracis tested, thus indicating its reliability as a tracer for that species. From the 60 strains of closely related Bacillus spp. examined, three bona fide B. cereus and one bona fide B. thuringiensis were found to harbour Ba813. This marker was also detected in Bacillus sp. isolates that were present at high levels in soil samples collected in a place where an anthrax outbreak had occurred. The significance and the possible function of the Ba813 locus is discussed.