Indigenous Bacteroides strains are closely associated with the occurrence and exacerbation of ulcerative colitis (UC). In this study, we aimed to clarify the effect of Bifidobacterium strains, another major member of colonic bacteria, on the development of gut inflammation using gnotobiotic mouse models associated with Bacteroides strains isolated from UC patients. Dextran sulfate (DSS) administration induced inflammation in the large intestine, in particular of the cecum, in the gnotobiotic mice associated with three strains of Bacteroides vulgatus, judging from the myeloperoxidase activity, occult blood score, and IgG leakage into the intestinal contents. However, the severity of the inflammation was greatly reduced in the gnotobiotic mice associated with both B. vulgatus and Bifidobacterium strains. The severity of the cecal inflammation was well correlated with the concentration of succinic acid in the cecum. Bacteriologically, the density of B. vulgatus strain A (BV-A) greatly decreased and the predominant strain changed from BV-A to BV-B on additional association with Bifidobacterium strains. Among gnotobiotic mice associated with a single B. vulgatus strain, the severity of cecal inflammation in BV-A-associated mice was greater than that in BV-B-associated mice. Each Bifidobacterium strain produced compound(s) more effectively inhibiting the growth of BV-A than BV-B in in vitro culture. Taken together, these results suggest that the severity of DSS-induced gut inflammation is closely associated with a particular B. vulgatus strain, and that Bifidobacterium strains may repress exacerbation of intestinal inflammation through growth inhibition of the B. vulgatus strain.