A representative mucilage, named Dioscorea-mucilage B, was isolated from the rhizophors of Dioscorea batatas DECAISNE. The final preparation was homogeneous as determined by ultracentrifugal analysis, polyacrylamide gel electrophoresis, and gel chromatography. Its water solution gave the intrinsic viscosity value of 21.0, and its molecular weight was estimated to be about 2000000. It was composed of partially acetylated mannan and of protein containing small amounts of phosphorus in a ratio of approximately 1.0 : 2.2. The polysaccharide moiety was isolated by treatment with Pronase followed by gel chromatography. Its molecular weight was estimated to be about 99000. Methylation and partial acetolysis studies showed that the polysaccharide is mainly composed of β-1→4 linked D-mannopyranose residues having about one fourth degree of branching at the C-3 positions. The O-acetyl groups were located at positions 6 and 2, 3, 6 of some of the mannose units.