Application of three successive Smith degradations to the water-soluble polysaccharide of Cladophora rupestris has confirmed both the highly branched nature of the molecule and the presence of 1,4-linked and end-group xylose. It has provided evidence of 1,6-linked and/or 6-sulphated galactofuranose units. Proof of mutually linked arabinose and galactose units has been obtained for the first time.