Two thermophilic, spore-forming strains, TM1(T) and TM5, were isolated from deep-sea sediment (4000 m below sea level) of the Ayu Trough in the western Pacific Ocean. Cells of the two strains were Gram-positive, motile and rod-shaped. Their spores were ellipsoidal, subterminal to terminal and occurred in swollen sporangia. The two strains grew at temperatures up to 65 degrees C and in the pH range 6.5-9.0. The NaCl concentration for optimal growth was 3.0 % (w/v) and growth was inhibited by 5 % (w/v) NaCl. Phylogenetic analysis based on 16S rRNA gene sequences indicated that strains TM1(T) and TM5 belonged to the genus Bacillus, and that strain TM1(T) was most closely related to Bacillus aeolius DSM 15084(T) (96.7 %), Bacillus smithii DSM 4216(T) (96.1 %), Bacillus methanolicus NCIMB 13113(T) (95.8 %) and Bacillus pallidus DSM 3670(T) (95.7 %). Between the 16S rRNA gene sequences of strains TM1(T) and TM5 there were only three nucleotide differences, implying that the two strains were of the same species. The cellular fatty acid profiles of the two strains were also very similar, with iso-C(15 : 0), iso-C(16 : 0), C(16 : 0), iso-C(17 : 0) and anteiso-C(17 : 0) as the major components. The G + C content of strain TM1(T) was 38.7 %. On the basis of phenotypic and molecular data, strains TM1(T) and TM5 represent a novel species of the genus Bacillus, for which the name Bacillus alveayuensis sp. nov. is proposed. The type strain is TM1(T) (= KCTC 10634(T) = JCM 12523(T)).