A novel, strictly anaerobic, non-motile, non-spore-forming, Gram-negative, short, straight rod with tapered ends, designated YIT 12065(T), was isolated from human faeces. Strain YIT 12065(T) was saccharolytic and negative for catalase, oxidase and urease, hydrolysis of aesculin and gelatin, nitrate reduction and indole production. The end products of glucose fermentation were acetic acid and a small amount of butyric acid. The DNA G+C content was 51.3 mol%. The predominant fatty acids were iso-C(15:0), C(16:0) and C(14:0). Respiratory quinones were not detected. The cell wall contained glutamic acid, serine, alanine and ll-diaminopimelic acid. The whole-cell sugars were ribose, rhamnose, galactose and glucose. Phylogenetic analyses based on 16S rRNA gene sequences using three treeing algorithms revealed that the strain formed a novel family-level lineage within the phylum Firmicutes, class Clostridia, order Clostridiales. Caldicoprobacter oshimai JW/HY-331(T) was shown to be the closest named relative on the basis of 16S rRNA gene sequence similarity (86.9%), followed by Tindallia californiensis DSM 14871(T) (86.3%) and Clostridium ganghwense JCM 13193(T) (86.1%). Similar 16S rRNA gene sequences (98.6-96.7%) were found amongst faecal uncultured clones of human and dugong (Dugong dugon). They clustered with strain YIT 12065(T) in a distinct and deep evolutionary lineage of descent in the order Clostridiales. The distinct phylogenetic position supports the proposal of Christensenella gen. nov., with the type species Christensenella minuta sp. nov. (type strain YIT 12065(T) =DSM 22607(T) =JCM 16072(T)). A new family Christensenellaceae fam. nov. is also proposed.