Methylation studies and partial acid hydrolysis have been carried out on the extracellular polysaccharide produced by the diatom Chaetoceros curvisetus. The polysaccharide contains the sugars galactose, rhamnose, and fucose and is partially sulfated. The presence of fucose in both furanose and pyranose forms within one molecule is reported for the first time. The fucofuranose is present as end groups, 1,2-linked and as branch points. Fucopyranose is mainly present as branch points in the polysaccharide, some being 1,3-linked, and a small fraction as end groups. The main part of rhamnose is 1,2-linked and galactose is mainly 1,3-linked.