A symbiotic thermophilic bacterium, strain SC-1, was isolated from hay compost (toebi) in Korea. The new isolate exhibited an obligate commensal interaction with a thermophilic Geobacillus strain and required crude extracts and/or culture supernatant from Geobacillus sp. SK-1 for axenic growth. The growth factors from Geobacillus sp. SK-1 were irreversibly inactivated by phenol or protease treatment, suggesting that they might be proteins. The cells of strain SC-1 were non-spore forming, nonmotile rods that were stained Gram-negatively. The isolate was a microaerophilic heterotroph. Growth was observed between 45 degrees and 70 degrees C (optimum: 60 degrees C; 2.4-h doubling time) and pH 6.0 and 9.0 (optimum: pH 7.5). The G+C content of the genomic DNA was 65 mol%, and the major quinones were MK-6 and MK-7. A phylogenetic analysis of its 16S rDNA sequence indicated that strain SC-1 is closely related to Symbiobacterium thermophilum and so was named Symbiobacterium toebii on the basis of its physiological and molecular properties.