Three Gram-positive, strictly anaerobic, non-spore-forming, rod-shaped organisms (strains YIT 12062(T), YIT 12063(T) and YIT 12064) were isolated from human faeces. Strain YIT 12062(T) was asaccharolytic and possessed a DNA G+C content of 58.3 mol%. Cells of strain YIT 12062(T) were negative for catalase, oxidase, urease, hydrolysis of aesculin and gelatin, nitrate reduction and indole production. Based on 16S rRNA gene sequence analysis, strain YIT 12062(T) was assigned to the genus Slackia (91.7-96.0 % sequence similarities to type strains of Slackia species). Biochemical data showed that the isolate was phenotypically distinct from all recognized species of the genus Slackia. Strain YIT 12062(T) therefore represents a novel species in the genus Slackia, for which the name Slackia piriformis sp. nov. is proposed. The type strain is YIT 12062(T) (=DSM 22477(T)=JCM 16070(T)). Following 16S rRNA gene sequence analysis, strains YIT 12063(T) and YIT 12064, which were isolated from different subjects, were shown to be most closely related to species of the genus Collinsella (93.8-95.1 % similarities to type strains). Although their phenotypic characteristics were very similar and they shared >99 % 16S rRNA gene sequence similarity and >97��1.8 % DNA-DNA relatedness, the two isolates could be discriminated by RAPD fingerprints. The DNA G+C contents of strains YIT 12063(T) and YIT 12064 were 60.8 and 61.0 mol%, respectively. They were saccharolytic in API test systems, positive for aesculin hydrolysis and negative for catalase, oxidase, urease, indole production, nitrate reduction and gelatin hydrolysis. The major end products of glucose fermentation of these strains were lactate, acetate and formate. Biochemical data supported the affiliation of strains YIT 12063(T) and YIT 12064 to the genus Collinsella and showed that they were phenotypically distinct from all recognized species of the genus Collinsella. Strains YIT 12063(T) and YIT 12064 therefore represent a novel species of the genus Collinsella, for which the name Collinsella tanakaei sp. nov. is proposed. The type strain is YIT 12063(T) (=DSM 22478(T)=JCM 16071(T)).