ABSTRACTXylan from Larchwood was utilized as a carbon source by three species ofBacteroidesfrom the human colon:Bacteroides ovatus, Bacteroides eggerthiiandBacteroides fragilissubsp. a. Xylan‐degrading enzymes of all three species were cell‐associated rather than extracellular, i.e. xylanase activity was detected in sonically disrupted bacteria but not in the extracellular fluid. The state of hydration of the xylan was an important factor in determining the extent to which the xylan was hydrolyzed by the bacterial enzymes. When xylan which had been partially hydrated by autoclaving was subjected to centrifugation at 17,000 xg for 20 min at 40C, a portion of the xylan was pelleted. This portion, which was presumably less well hydrated than the xylan which remained in solution, was not digested at all by the bacterial enzymes. By contrast, xylan which remained soluble after centrifugation (soluble xylan) was degraded by bacterial xylanases. Xylose was the main product of digestion, although traces of arabinose, glucose and higher oligomers of xylose were also detected. Even after exhaustive hydrolysis, less than 60% of the soluble xylan was hydrolyzed. Removal of most of the arabinose branches did not increase the extent of hydrolysis. Moreover, the carbohydrate composition of the portion of the soluble xylan which resisted hydrolysis was the same as the composition of the xylan which was hydrolyzed. Thus it is likely that hydrolysis was limited by aggregate formation rather than by structural features such as arabinose branc