A novel butyrate-producing bacterium, strain 1y-2(T), was isolated from a stool sample of a 1-year-old, healthy Dutch infant. The isolate was obtained by using lactate and acetate as sources of carbon and energy. The strain was Gram-variable, strictly anaerobic and spore-forming and formed curly rod-shaped cells that fermented glucose into butyrate, lactate, formate and acetate as main products. The DNA G+C content of the strain was 44.5 mol% and its major cellular fatty acids were C12:0, iso-C19:1 I and C16:0. Strain 1y-2(T) was related to Anaerostipes caccae DSM 14662(T) based on 16S rRNA gene sequence analysis, with 3% divergence, but hybridization studies of their genomic DNA revealed only 33% relatedness. Moreover, strain 1y-2(T) showed marked physiological and biochemical differences from known species of the genus Anaerostipes. Based on phylogenetic, chemotypic and phenotypic criteria, we propose that strain 1y-2(T) should be classified in the genus Anaerostipes within a novel species, Anaerostipes rhamnosivorans sp. nov. The type strain is 1y-2(T) ( = DSM 26241(T) = KCTC 15316(T)).