Purified extracellular endo-1,4-β-xylanase (EC 184.108.40.206) of the yeast Cryptococcus albidus was found to catalyze not only the known 1,4-β-transfer, but an alternative transglycosylation reaction leading to the formation of 1,3-β-glycosidic linkages. From a mixture of products of β-xylanase degradation of phenyl β-D-xylopyranoside three xylooligosaccharide fractions, differring chromatographically from the 1,4-β-linked products, were isolated by preparative paper chromatography. Their structure was elucidated by mass spectrometry, 13C-NMR spectroscopy and enzymic hydrolysis by β-xylanase and β-xylosidase. The isomeric xylotriose was identified as 3-O-β-D-xylopyranosyl-4-O-β-D-xylopyranosyl-D-xylose. The fraction of isomeric tetrasaccharides was found to be represented mainly by 4-O-β-D-xylopyranosyl-3-O-β-D-xylopyranosyl-4-O-β-D-xylopyranosyl-D-xylose. The xylooligosaccharides containing one 1,3-β-linkage were also produced on the enzyme treatment of 1,4-β-xylotriose and 1,4-β-xylan. When treated with the enzyme responsible for their synthesis, the isomeric xylooligosaccharides were hydrolyzed at the 1,3-β-linkage, despite the fact the enzyme does not attack 1,3-β-xylan. The results are interpreted in the relation to the characterized four-subsite substrate-binding site of the enzyme.