A -D-glucosidase has been purified to apparent homogeneity from the cotyledons of germinated nasturtium (Tropaeolum majus L.) seedlings during the mobilization of the xyloglucan stored in the cotyledonary cell walls. The purified protein (Mr 76 000; a glycoprotein; pI > 9.5; apparent pH optimum 4.5; temperature optimum 30°C) catalysed the hydrolysis of p-nitrophenyl--D-glucopyranoside, cello-oligosaccharides, -linked glucose disaccharides, and certain xyloglucan oligosaccharides. Glucose disaccharides with different linkages were hydrolysed at different rates [(1ν3) > (1ν4) > (1ν2) > (1ν6)] with significant transglycosylation occurring in the early stages of the reaction. Cello-oligosaccharide hydrolysis was also accompanied by extensive transglycosylation to give transitory accumulations of higher oligosaccharides. At least some of the glycosyl linkages formed during transglycosylation were (1ν6)-. Xyloglucan oligosaccharides xylose-substituted at the non-reducing terminal glucose residue (XXXG, XXLG, XLXG and XLLG, where G is an unsubstituted glucose residue, X is a xylose-substituted glucose residue, and L is a galactosylxylose-substituted glucose residue) were not hydrolysed. Some xyloglucan oligosaccharides with an unsubstituted non-reducing terminal glucose residue (GXXG, GXLG and GXG) were hydrolysed, but others (GLXG and GLLG) were not. This indicated steric hindrance by L but not X substitution at the glucose residue next to the one at the non-reducing end of the oligosaccharide. Hydrolysis of xyloglucan oligosaccharides was not accompanied by transglycosylation. Natural xyloglucan subunit oligosaccharides (XXXG, XXLG, XLXG, XLLG) were totally degraded to their monosaccharide components when treated with nasturtium -D-galactosidase ( Edwards et al. (1988 ) J. Biol. Chem. 263, 4333–4337), followed by alternations of nasturtium xyloglucan-specific α-xylosidase ( Fanutti et al. (1991 ) Planta 184, 137–147) and this enzyme. Several extensively overlapping cDNA clones were obtained by RT–PCR and by screening cDNA libraries. A composite, full-length DNA had an open reading frame of 1962 bp, encoding a polypeptide of 654 amino acids, including all N-terminal and internal sequences obtained from the purified -glucosidase protein, and a motif resembling plant signal sequences thought to direct proteins to the cell wall. Database searches revealed homology with -glucosidases from several sources (plant, bacteria, yeast), notably with glycosylhydrolases of ‘Family 3’, according to the classification of Henrissat ( Henrissat (1991) Biochem. J. 280, 309–316). There was strong sequence homology with a -glucan exo-hydrolase from barley ( Hrmova et al. (1996 ) J. Biol. Chem. 271, 5277–5286). The nasturtium -glucosidase is ascribed a role in xyloglucan mobilization, and its interaction with the α-xylosidase and the -galactosidase is modelled.