A purified proteo-heteroglycan, [α]D+72.5° was isolated from Piricularia oryzae, a pathogenic fungus of rice blast disease (Imochi-byo), by means of hot citrate buffer extraction, cetavlon fractionation, and DEAE-Sephadex chromatography. It was found to be homogeneous by electrophoresis and by analytical ultracentrifugation to have an s value 6.1 and to contain 91% (w/w) of carbohydrate, which consists of D-mannose, D-glucose, and D-galactose in a molar ratio of 6 : 2 : 1. Partial acid hydrolysis and methylation analysis of the carbohydrate moiety of the proteo-heteroglycan indicate that the molecule is composed of mannan, the side chain terminals of which are partially modified with D-glucopyranose and D-galactofuranose. Enzymatic hydrolysis with bacterial α-D-mannanase has been shown to remove most of the side chains from the heteroglycan, leaving an (1–6) linked mannan back-bone with a small amount of side chains, the terminals of which must be modified with D-glucopyranose or D-galactofuranose. The carbohydrate to protein linkage of the proteo-heteroglycan was shown by alkaline β-elimination, to be mannosyl serine or mannosyl threonine.