A strictly anaerobic, mesophilic, endospore-forming bacterium, designated strain HY-35-12T, was isolated from a soil sample in Jeju, Korea. Cells of this isolate were Gram-positive, motile rods that formed oval to spherical terminal spores. Strain HY-35-12T grew optimally at 30 degrees C, pH 7.0 and 0-0.5 % (w/v) NaCl. The isolate produced pyruvate, lactate, acetate, formate and hydrogen as fermentation end products from glucose. The G + C content of DNA of the isolate was 41 mol%. Phylogenetic analysis based on 16S rRNA gene sequences revealed that the organism formed a monophyletic clade with Clostridium xylanovorans and Clostridium aminovalericum in cluster XIVa of the genus Clostridium. The closest phylogenetic neighbour was C. xylanovorans, with 96.65 % 16S rRNA gene sequence similarity. Several physiological and chemotaxonomic properties were identified that enable strain HY-35-12T to be distinguished from phylogenetically related clostridia. On the basis of polyphasic characteristics, it is proposed that strain HY-35-12T (= IMSNU 40003T = KCTC 5026T = DSM 15929T) represents a novel species, Clostridium jejuense sp. nov.