The structure of the polysaccharides extracted from New Zealand samples of Grateloupia was examined by 13 C NMR spectroscopy and by chemical means, and was found to consist of alternating 3-linked and 4-linked residues. All Grateloupia polysaccharides had low levels of 2- O -methyl-3,6-anhydro-L-galactose, and variable levels of 6- O -methyl-D-galactose. The major diads of Grateloupia urvilleana polysaccharide were non-sulfated 3-linked galactose with 4-linked D-galactose-2-sulfate, and 3-linked galactose-2-sulfate with non-sulfated 3,6-anhydro-D-galactose. In Grateloupia stipitata and Grateloupia prolifera polysaccharides, approximately half the 4-linked units were in the L-form, and the diad comprising 3-linked D-galactose-2-sulfate and 4-linked L-galactose-2,3-disulfate, an entity previously found mainly in Champia species, was also present. The diads with 4-linked galactose in the D-form comprised 4-linked D-galactose or 3,6-anhydro-D-galactose, and these diads had the sulfation found in Grateloupia urvilleana , except that a large proportion of these diads had no sulfation at all. Although these polysaccharides contain the same sugars as Pachymenia polysaccharides, apart possibly from the diad comprising 3-linked galactose-2-sulfate and 3,6-anhydro-L-galactose, which is a minor component of the Grateloupia polysaccharides, the polysaccharides from the two genera have no diads in common.