16s rRNA gene sequences of the type strains of 11 species belonging to the Bacillus brevis and Bacillus aneurinolyticus groups were determined. On the basis of the results of gene sequence analyses, these species were separated into two clusters. The B. brevis cluster included 10 species, namely, Bacillus brevis, Bacillus agri, Bacillus centrosporus, Bacillus choshinensis, Bacillus parabrevis, Bacillus reuszeri, Bacillus formosus, Bacillus borstelensis, Bacillus luterosporus, and Bacillus thermoruber. Bacillus aneurinolyticus and Bacillus migulunus belonged to the B. aneurinolyticus cluster. Moreover, the two clusters were phylogenetically distinct from other Bacillus, Amphibacillus, Sporoluctobacillus, Paenibacillus, and Alicyclobacillus species. On the basis of our data, we propose reclassification of the B. brevis cluster as Brevibacillus gen. nov. and reclassification of the B. aneurinolyticus cluster as Aneurinibacillus gen. nov. By using 16s rRNA gene sequence alignments, two specific PCR amplification primers were designed for differentiating the two new genera from each other and from other aerobic, endospore-forming organisms.The aerobic, rod-shaped, endospore-forming genus Bacillus is a systematically diverse taxon (5). The members of this genus exhibit a wide range of DNA base compositions, and the major amino acid compositions of the cell walls of these organisms vary (6, 22, 32). Analyses of 16s rRNA gene sequences have identified at least eight phylogenetic groups in the genus Bacillus (2,3,7,20,(22)(23)(24)33,38). Two of these groups have been reclassified as new genera. One genus, the genus Alicyclobacil-.lus (38), consists of thermoaciduric species that contain rarely encountered cellular o-cyclic fatty acids. The other new genus, the genus Paenibacillus (3), was distinguished on the basis of the results of slot blot hybridization in which a specific probe was used.Recent taxonomic studies have shown that strains previously assigned to Bacillus brevis should be separated into nine species, namely, Bacillus brevis, Bacillus agri, Bacillus centrosporus, Bacillus choshinensis, Bacillus parabrevis, Bacillus reuszeri, Bacillus fonnosus, Bacillus borstelensis, and Bacillus migulanus (18,19, 28,29,34). A closely related species, Bacillus aneurinolyticus, has also been revived by Shida et al. (30). On the basis of the results of comparisons of their phenotypic characteristics, chemosystematic profiles, and conserved specific Slayer proteins, the 10 species mentioned above were separated into two groups (28-30, 34). One group, designated the Bacillus brevis group, included all of the species derived from Bacillus brevis except Bacillus miplanus. The other group, called the Bacillus aneurinolyticus group, included Bacillus aneurinolyticus and Bacillus migulanus. These groups corresponded closely to the groups generated by a numerical analysis of electrophoretic whole-cell protein patterns (31). Moreover, the results of phylogenetic studies demonstrated that Bacillus laterosporus and Bacillus thermoruber f...