A novel Gram-stain-positive, motile, catalase- and oxidase-positive, endospore-forming, facultatively anaerobic rod, designated strain JSM 100118^T, was isolated from an oyster (Crassostrea hongkongensis) collected from the tidal flat of Naozhou Island in the South China Sea. Strain JSM 100118^T was able to grow with 0–13 % (w/v) NaCl (optimum 2–5 %), at pH 5.5–10.0 (optimum pH 7.5) and at 5–50 °C (optimum 30–35 °C). The cell-wall peptidoglycan contained meso-diaminopimelic acid as the diagnostic diamino acid. The predominant respiratory quinone was menaquinone-7 and the major cellular fatty acids were anteiso-C_{15 : 0}, iso-C_{15 : 0}, C_{16 : 0} and C_{16 : 1}ω11c. The polar lipids consisted of diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylglycerol, an unknown glycolipid and an unknown phospholipid. The genomic DNA G+C content was 35.9 mol%. Phylogenetic analysis based on 16S rRNA gene sequences indicated that strain JSM 100118^T belonged to the genus Bacillus , and was most closely related to Bacillus litoralis SW-211^T (98.9 % 16S rRNA gene sequence similarity), Bacillus halosaccharovorans E33^T (98.3 %), Bacillus niabensis 4T19^T (97.8 %) and Bacillus herbersteinensis D-1,5a^T (97.1 %). The combination of results from the phylogenetic analysis, DNA–DNA hybridization, and phenotypic and chemotaxonomic characterization supported the conclusion that strain JSM 100118^T represents a novel species of the genus Bacillus , for which the name Bacillus crassostreae sp. nov. is proposed. The type strain is JSM 100118^T ( = CCTCC AB 2010452^T = DSM 24486^T = JCM 17523^T).