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Mannitol constitutes the major carbohydrate of low molecular weight in the brown seaweeds named in the title; glucose is also present in Himanthalia, and glucose and myo-inositol were detected in Bifurcaria. Laminarin, alginic acid, “fucans”, and cellulose were separated and characterised from each of the species investigated. The “fucans”, which were present in five sequential extracts obtained with different extractants, comprised variable proportions of fucose, xylose, glucuronic acid, galactose (traces), and half-ester sulphate. Fractionation on DE-cellulose led to the isolation of highly sulphated materials having a high content of fucose, polymers having a high content of glucuronic acid and a low content of sulphate, and polysaccharides with proportions of sugars and sulphate between these two extremes. It is concluded that all three seaweeds synthesise a wide spectrum of these polysaccharides.