A new oligosaccharide subunit of xyloglucan was isolated from the beta-(1----4)-endoglucanase digestion products of the xyloglucan in what is referred to as "sycamore extracellular polysaccharides" and found to be an undecasaccharide having two terminal alpha-L-fucopyranosyl residues. The undecasaccharide was structurally characterized by 1H-n.m.r. spectroscopy, fast-atom bombardment mass spectrometry (f.a.b.-m.s.), and glycosyl-residue and glycosyl-linkage composition analyses. The structure of the undecasaccharide was confirmed by digesting it with a hydrolase that releases alpha-D-Xylp-(1----6)-D-Glc from the non-reducing end of xyloglucan oligosaccharides.