Two newly isolated strains of obligately anaerobic bacteria from human faeces are shown here to be related to Fusobacterium prausnitzii, which is regarded as one of the most abundant colonizers of the human colon. These strains, along with Fusobacterium prausnitzii ATCC 27768(T) and 27766, are non-motile and produce butyrate, formate and lactate, but not hydrogen as fermentation products. A new finding is that all four strains produce D-lactate, but not L-lactate. The strains have a requirement for acetate in the growth medium and this may account for the previously reported requirement for rumen fluid. The DNA G+C content of the four strains is 47-57 mol%. Together with phylogenetic analysis based on 16S rRNA sequencing, this establishes that Fusobacterium prausnitzii strains are only distantly related to Fusobacterium sensu stricto and are more closely related to members of Clostridium cluster IV (the Clostridium leptum group). It is proposed that a new genus, Faecalibacterium gen. nov. be created; this genus should include Faecalibacterium prausnitzii gen. nov., comb. nov. ATCC 27768(T) (= NCIMB 13872(T)) (formerly Fusobacterium prausnitzii) as the type species together with ATCC 27766 and the newly isolated strains A2-165 and L2-6.