Abstract 1. The mucilage of Chondrus ocellatus Holmes has been investigated by the partial methanolysis method. 2. A new disaccharide, the structure of which is shown to be 4-O-β-d-galactopyranosyl-3,6-anhydro-d-galactose, has been isolated in the form of its dimethylacetal and methyl β-glycoside in a good yield. 3. The disaccharide has been named carrabiose. 4. It has been suggested that 1→3′ linked carrabiose represents the repeating unit for the most part of the carbohydrate of the Chondrus mucilage.