We isolated several strains from various clinical samples (five samples of blood, four of intra-abdominal pus and one of infected soft tissue) that were anaerobic, motile or non-motile and Gram-positive rods. Some of the strains formed spores. Phylogenetic analysis of the 16S rRNA gene sequence showed that these organisms could be placed within clostridial cluster IV as defined by Collins et al. [(1994). Int J Syst Bacteriol 44, 812-826] and shared more than 99 % sequence similarity with Clostridium orbiscindens DSM 6740(T) and Eubacterium plautii DSM 4000(T). Together, they formed a distinct cluster, with Bacteroides capillosus ATCC 29799(T) branching off from this line of descent with sequence similarities of 97.1-97.4 %. The next nearest neighbours of these organisms were Clostridium viride, Oscillibacter valericigenes, Papillibacter cinnamivorans and Sporobacter termitidis, with sequence similarities to the respective type strains of 93.1-93.4, 91.2-91.4, 89.8-90 and 88.7-89.3 %. On the basis of biochemical properties, phylogenetic position, DNA G+C content and DNA-DNA hybridization, it is proposed to unify Clostridium orbiscindens and Eubacterium plautii in a new genus as Flavonifractor plautii gen. nov., comb. nov., with the type strain Prévot S1(T) (=ATCC 29863(T) =VPI 0310(T) =DSM 4000(T)), and to reassign Bacteroides capillosus to Pseudoflavonifractor capillosus gen. nov., comb. nov., with the type strain CCUG 15402A(T) (=ATCC 29799(T) =VPI R2-29-1(T)).