A mesophilic, strictly anaerobic bacterium, strain Sjm18-20(T), was isolated from the alimentary canal of a Japanese corbicula clam. Cells of strain Sjm18-20(T) were Gram-negative, non-sporulating, straight to slightly curved rods, 2.5-6.0 microm long, and were motile with oscillatory movements by means of peritrichous flagella. Cells elongated to 30 microm after prolonged cultivation. Optimum growth was observed at 30 degrees C and pH 6.0-6.5. Growth occurred below 4.0 % (w/v) NaCl. Strain Sjm18-20(T) produced acid from D-glucose and a few pentoses such as L-arabinose, D-ribose and D-xylose. n-Valeric acid was the major end product from glucose. The genomic DNA G+C content of strain Sjm18-20(T) was 52.9 mol%. Phylogenetic analysis based on the 16S rRNA gene revealed that strain Sjm18-20(T) could be accommodated in clostridial cluster IV of the low-G+C-content Gram-positive bacteria and that the closest neighbour of this organism (92.6-92.9 % similarity) was the cloned 16S rRNA gene sequence of a not-yet cultured bacterium, thought to represent Oscillospira guilliermondii. The nearest cultivated neighbours of strain Sjm18-20(T) were Clostridium orbiscindens DSM 6740(T) and Clostridium viride T2-7(T), with sequence similarities of 91.3 and 89.1 %, respectively. On the basis of phenotypic features and phylogenetic position, it is proposed that this isolate represents a novel species in a new genus, Oscillibacter valericigenes gen. nov., sp. nov. The type strain of Oscillibacter valericigenes is Sjm18-20(T) (=NBRC 101213(T) =DSM 18026(T)).