The galactomannan-protein (GMP) containing 89% of carbohydrate and 11% of protein was purified from the hyphal walls of Aspergillus oryzae. The polymer contains both N- and O-linked carbohydrate chains. Mild alkali treatment of the polymer yielded mannose and alpha-1,2-mannobiose. The alkali-resistant polymer was composed of 14% of protein and 86% of carbohydrate containing mannose and galactose with the ratio of 5:3. Mild acid treatment of the alkali-resistant polymer caused a great change in optical rotation from -15 degrees to +63 degrees with complete removal of galactose residues, and the residual polymer was a mannoprotein of which side chains were completely hydrolyzed by a bacterial alpha-1,2-mannosidase. The structural analyses with H-1- and C-13-NMR spectroscopies of native, alkali-degraded, alkali-acid degraded polymers and their acetolysis products indicate that the polysaccharide part of the polymer has an alpha-1,6-linked mannan backbone with side chain consisting of single mannose residues linked via an alpha-1,2 bond and almost all of the side chain consisting of nonreducing terminal mannose residues are substituted by beta-galactofuranose residues.