Using a polyphasic approach, a taxonomic study was performed on seven strains of an unknown Gram-reaction-positive, non-spore-forming, obligately anaerobic coccus-shaped bacterium, isolated from a swine-manure storage pit. Comparative 16S rRNA gene sequencing confirmed that all seven isolates were highly related to each other and formed a hitherto unknown lineage within the clostridial rRNA XI cluster of organisms. Pairwise analysis demonstrated that the novel organism was most closely related to Peptostreptococcus anaerobius CCUG 7835(T) and Peptostreptococcus stomatis CCUG 51858(T) with 16S rRNA gene sequence similarities of 95.5 and 93.0 %, respectively. The peptidoglycan type of the cell wall was determined to be A4α l-Lys-d-Asp and glucose, xylose and traces of mannose were detected as the cell-wall sugars. Based on biochemical, chemotaxonomic and phylogenetic evidence the unknown bacterium represents a new species of the genus Peptostreptococcus, for which the name Peptostreptococcus russellii sp. nov, is proposed. The type strain is RT-10B(T) ( = CCUG 58235(T) = NRRL B-59380(T) = DSM 23041(T)).