A Gram-positive, rod-shaped, spore-forming bacterium, strain S-11T, was isolated from the activated sludge of a Bacillus-dominated wastewater treatment system in South Korea and was characterized using a polyphasic approach in order to determine its taxonomic position. Cells (0.5-0.6 x 2.0-2.2 microm) were motile by means of a single subpolar flagellum. They bore ellipsoidal endospores that lay in a central position in swollen sporangia. Phylogenetic analysis based on 16S rRNA gene sequences revealed that strain S-11T was a member of the genus Oceanobacillus. 16S rRNA gene sequence similarity values and DNA-DNA relatedness of strain S-11T to the type strains of other Oceanobacillus species were less than 96.2 and 66.0 %, respectively. Strain S-11T showed distinct differences in the G+C content of the genomic DNA (33.6 mol%). The major cellular fatty acids were iso-C14 : 0, iso-C15 : 0, anteiso-C15 : 0 and iso-C16 : 0. The major isoprenoid quinone was MK-7. There were also some physiological differences in comparison with the type strains of Oceanobacillus species: tests for production of acetoin and acid production from dulcitol, erythritol, myo-inositol and sorbitol were positive. The results of DNA-DNA hybridization and physiological and biochemical tests allowed genotypic and phenotypic differentiation of strain S-11T from the six Oceanobacillus species and subspecies with validly published names. Strain S-11T therefore represents a novel species, for which the name Oceanobacillus caeni sp. nov. is proposed, with the type strain S-11T (=KCTC 13061T =CCUG 53534T =CIP 109363T).