Strains of Selenomonas ruminantium vary considerably in their capacity to ferment xylooligosaccharides. This ability ranges from strain GA192, which completely utilized xylose through xylotetraose and was able to ferment considerable quantities of larger oligosaccharides, to strain HD4, which used only the simple sugars present in the hydrolysate. The ability of S. ruminantium GA192 to utilize xylooligosaccharides was correlated with the presence of xylosidase and arabinosidase activities. The production of these activities appears to be regulated in response to carbon source used for growth. Both arabinosidase and xylosidase were induced by growth on xylose or xylooligosaccharides, but no activity was detected in glucose-or arabinose-grown cultures. A genetic locus from S. ruminantium GA192 was cloned into Escherichia coli JM83 that produced both xylosidase and arabinosidase activities. Analyses of crude extracts from the E. coli clone and S. ruminantium GA 192 by using native polyacrylamide gel electrophoresis and methylumbelliferyl substrates indicated that a single protein was responsible for both activities. The enzyme expressed in E. coli was capable of degrading xylooligosaccharides derived from xylan. DNA sequencing of the locus demonstrated the presence of an open reading frame that encodes for a protein of 61,174 molecular weight.