Four bacterial strains, designated ST18(T), HM244, HM250 and DI49, were isolated from the fresh faeces of four thoroughbred horses in Japan. Cells were Gram-positive, strictly anaerobic, catalase-negative, non-spore-forming, non-motile rods that occurred in chains. They were placed in the same subcluster based on 16S rRNA gene sequence analysis, phenotypic characteristics and levels of DNA-DNA relatedness. Their DNA G+C content ranged from 36 to 38 mol%. Lactobacillus catenaformis, Lactobacillus vitulinus and Catenibacterium mitsuokai belong to cluster XVII of the Clostridium subphylum. Strain ST18(T) was most closely related to L. catenaformis ATCC 25536(T) in the phylogenetic tree, but these strains shared only 89.9 % (1336/1486 bp) 16S rRNA gene sequence similarity. L. catenaformis, L. vitulinus and C. mitsuokai are homofermentative bacteria, whereas ST18(T) produced CO(2) from glucose. Whereas the cell-wall peptidoglycan type of L. catenaformis and L. vitulinus wasL-Lys-L-Ala(3), that of C. mitsuokai and the subgroup represented by ST18(T) was A1gamma (L-Ala-D-Glu-meso-diaminopimelic acid). On the basis of 16S rRNA gene sequence divergence of more than 10 % from L. catenaformis as well as phenotypic characteristics, strains ST18(T), HM244, HM250 and DI49 are considered to represent a novel species of a new genus belonging to the Clostridium subphylum cluster XVII, for which the name Sharpea azabuensis gen. nov., sp. nov. is proposed. The type strain of Sharpea azabuensis is ST18(T) (=JCM 14210(T) =DSM 18934(T)).