Abstract The polysaccharide extracted from the red seaweed Gigartina tenella has been separated into κ- and λ-polysaccharide by potassium chloride. The former polysaccharide is a gelling component precipitated with potassium chloride, while the latter is a non-gelling component insensitive to the precipitant. Complete hydrolysis and methanolysis, together with quantitative determination, have shown that the κ-polysaccharide is composed mainly of d-galactose, 3, 6-anhydro-d-galactose, and an ester sulfate in the molar proportion 1 : 0.98 : 1.17. Very small amounts of l-galactose and d-xylose have also been isolated from the products of hydrolysis. The partial methanolysis of the κ-polysaccharide yielded dimethyl acetal of carrabiose (4-O-β-d-galactopyranosyl-3, 6-anhydro-d-galactose) in a good yield. It follows from these results that this disaccharide is a chief repeating unit of the carbohydrate moiety of the polysaccharide, as in the case of κ-carrageenan.