The production of Pleurotus spp. has been increasing worldwide at a rapid rate. These mushrooms have attracted great attention owing to their good source of nonstarchy carbohydrates, having a high content of dietary fiber, and containing moderate quantities of proteins with most of the essential amino acids, mineral and vitamins. They have been shown to modulate the immune system, have hypoglycemic activity, and to inhibit tumor growth. We now isolate two polysaccharides from the edible basidiomycete Pleurotus eryngii. They were obtained via successive aqueous extraction, freeze-thawing, precipitation with Fehling solution, and ultrafiltration, and then investigated using ¹³C and ¹H-nuclear magnetic resonance spectroscopy (including DEPT, TOCSY, COSY, and HMQC techniques) and methylation analysis. One was a linear partially 3-O-methylated (1®6)-linked galactan containing Gal and 3-O-Me-Gal, in a 3:1 molar ratio. The other was a mannogalactan, containing Man, Gal and 3-O-Me-Gal, in a 31:17:52 molar ratio, which had the same main chain, a part of these units being substituted in the position O-2 with b-D-mannopyranose residues. In basidiomycetes, the polysaccharides that have been found in the fruiting body were mainly glucans and heterogalactans. Thus we have isolated a galactan which has not been previously described and a mannogalactan that was similar those found in Pleurotus spp, with differences only in its proportion of 3-O-Me-a-D-galactopyranosyl residues. The presence of partially 3-O-methylated polysaccharides appears to be typical of Pleurotus spp.