The pathogenesis of ulcerative colitis (UC) remains unknown. It is thought to be due to an abnormal and uncontrolled immune response to normally occurring constituents of the intestine. Microbial agents appear to be involved in the pathogenesis and intestinal bacteria seem to be an important factor in the development and chronicity. The aim of this study was to investigate the colonic microbiota of a patient with UC. The colonic tissues were taken during surgery from a 12-year-old girl suffering from UC. The microbiota on the colonic samples was studied by cloning and sequencing of amplified 16S rRNA genes. Compared with healthy subjects, alteration of the dominant bacterial group was observed in the UC patient. We found a high incidence of Enterobacteriaceae, Bacteroides fragilis, and the single phylotype of the Faecalibacterium prausnitzii-like "Butyrate-producing bacterium" L2-6. Furthermore, there was a substantial presence of Pseudomonas aeruginosa in the present case of UC. The high proportion of adverse proinflammatory species is striking in the present case compared with more normal situations. Even if those bacteria are not the cause of the UC, they most probably enhance the symptoms of the disease.