AimsZ: To characterize fibre degradation, colonization and fermentation, and xylanase activity of two xylanolytic bacteria Bacteroides xylanisolvens XB1AT and Roseburia intestinalis XB6B4 from the human colon. Methods and Results: The bacteria grew well on all the substrates chosen to represent dietary fibres: wheat and corn bran, pea, cabbage and leek fibres, and also on purified xylans. Roseburia intestinalis colonized the substrates more efficiently than Bact. xylanisolvens. For the two bacteria, 80-99% of the total xylanase activity was associated with the cells whatever the substrate and time of growth. Optimal specific activities of cells were obtained on oat spelt xylan; they were higher than those previously measured for xylanolytic bacteria from the human gut. Roseburia intestinalis produced high molecular mass xylanases (100-70 kDa), while Bact. xylanisolvens produced lower molecular mass enzymes, including a cell-associated xylanase of 37 kDa. Conclusions: The two bacteria display very high xylanolytic activity on the different substrates. Differences were observed on substrate attachment and enzyme systems, suggesting that the two species occupy different niches within the gut microbiota. Significance and Impact of the Study: This study characterizes xylan degradation by two major species of the human intestine.