A strictly anaerobic bacterial strain (WN081^T) was isolated from rice-straw residue in a methanogenic reactor treating waste from cattle farms in Japan. Cells were Gram-staining negative, non-motile, non-spore-forming straight rods. The strain grew rather well on PY agar slants supplemented with a B-vitamin mixture as well as sugars (PYV4S medium) and made translucent and glossy colonies. Growth in liquid medium with the same composition, however, was scanty, and growth was not improved in spite of various additives to the medium. Strain WN081^T produced small amounts of acetate, propionate, isobutyrate, butyrate, isovalerate and H₂ from PYV liquid medium. The strain did not use carbohydrates or organic acids. The pH range for growth was narrow (pH 6.8−8.2), having a pH optimum at 6.8−7.5. The temperature range for growth was 10−37°C, the optimum being 25−30°C. The strain was sensitive to bile, and did not have catalase or oxidase activities. Hydrogen sulfide was produced from L-cysteine and L-methionine as well as peptone. Indole was produced from L-tryptophan and peptone. The strain had iso-C_15:0 as the exclusively predominant cellular fatty acid (70%) together with some branched chain components (such as iso-C_15:0 DMA, iso-C_17:0 3-OH and iso-C_15:0 aldehyde) as minor components. The genomic DNA G+C content was 32.3 mol%. Phylogenetic analysis based on the 16S rRNA gene sequence placed strain WN081^T in the phylum Bacteroidetes with rather low sequence similarities with the related species such as Rikenella microfusus (85.7% sequence similarity), Alistipes putredinis (85.5%) and Alistipes finegoldii (85.5%) in the family Rikenellaceae. Based on the phylogenetic, physiological and chemotaxonomic analyses, the novel genus and species Anaerocella delicata gen. nov., sp. nov. is proposed to accommodate the strain. The type strain is WN081^T (= JCM 17049^T = DSM 23595^T).