To investigate the adhesion factor of Bacteroides vulgatus derived from ulcerative colitis (UC), we isolated B. vulgatus strains from the large intestinal mucosa of UC patients and non-UC individuals, and examined their adherence to tissue-cultured cells. The adherence to tissuecultured cells in UC-derived strains (36.5 ± 7.9 %) was higher than that in non-UC-derived strains (13.2 ± 7.7 %). PCR and sequencing analysis of outer membrane proteins revealed that the strains derived from five of seven (71.4 %) UC patients had ompA genes belonging to either ompA variant type A or B. The adherence rates in Escherichia coli DH5α transformants with ompA type A variants (33.3 ± 4.6 %) and type B variants (34.6 ± 7.1 %) were higher than the rate in those with non-UC ompA (16.4 ± 4.0 %). Our results suggest that B. vulgatus isolates in the mucosal flora of the large intestine of UC patients have a high frequency of ompA variants and that the variation of ompA variants is one of the factor causing an increase in the adherence of the bacterium.