Water-soluble polysaccharide from ascocarps of Cordyceps sinensis was purified by ethanol fractionation and gel filtration. Purified polysaccharide, CS-I, [α] D-45.3°(c=1, H2O), gave D-galactose (phenylosazone, mp 185-187°) and D-mannose (phenyl-hydrazone, mp 199-200°) in a molar ratio of 1 : 1 by acid hydrolysis. From the results of periodate oxidation, Smith-type degradation, methylation analysis, partial acid hydrolysis, and 13C-NMR spectrometry, it is concluded that CS-I is a highly branched galactomannan which consists of mannan core and galactosyl oligomer containing branches. The mannan core mainly contains (1→2)-α-linked-D-mannopyranosyl residues, and the branches contain (1→3), (1→5), and (1→6)-linked-D-galactofuranosyl, (1→4)-linked-D-galactopyranosyl residues. The non-reducing ends are D-galactofuranose and D-manno-pyranose, and the branching points are D-mannopyranosyl residues.