As part of a culturomics study to identify bacterial species associated with inflammatory bowel disease, a large collection of bacteria was isolated from patients with ulcerative colitis. Two of these isolates were tentatively identified as members of the family . Following phylogenetic analysis based on 16S rRNA gene sequence and genome sequences, both strain 128 and 539 were found to be most closely related to , with G+C contents of 48.6 and 50.5 mol%, respectively, and the genome sizes of 2 864 314 and 2 580 362 base pairs, respectively. Strains 128 and 539 were strict anaerobe rods that grew in long chains between 37 and 42 °C. Scanning electron microscopy did not reveal flagella, fimbriae or visible endospores. Biochemical analysis showed nearly identical results for both strains with enzymatic activity of C4 and C8 esterases, acid phosphatase, naphthol-AS-BI-phosphohydrolase, β-glucuronidase, -acetyl-β-glucosaminidase and arginine arylamidase. In addition, both strains produced indole and reduced nitrate. Major fatty acids were identified as C 9 (oleic acid, 64.06% in 128 and 74.35% in 539), C 7/C 9t/C 12t/UN17.834 (16.18 % in 128 and 6.22% in 539) and C (6.23% in 128 and 7.37% in 538). Based on these analyses two novel species are proposed, sp. nov. with the type strain 128 (=NCTC 14626=DSM 112815) and sp. nov. with the type strain 539 (=NCTC 14627=DSM 112814).