The highly complex polysaccharide extracted from the New Zealand red alga Pachymenia lusoria (Grev.) J. Ag. has been characterised and certain structural features defined. A reductive hydrolysis procedure was used for constituent sugar and linkage analyses, with trideuteriomethylation being employed to enable the location of natural methyl ether groups to be determined. A reductive partial-hydrolysis procedure allowed agarobiosyl constituent residues to be identified. The analytical results are consistent with the polymer having a linear backbone of 3-1inked D-galactopyranosyl alternating with 4-1inked D-or L-galactopyranosyl residues. The 3-1inked residues are nearly all 2-sulfated, with 1 in 3 also being 6-O-methylated and 1 in 5 also bearing a 4,6-pyruvic acetal residue. About one-third of the polymer is comprised of blocks of agarobiosyl repeat units that are 2-sulfated on the /3-D-galactopyranosyl and one-third 2-O-methylated on the 3,6-anhydro-L-galactosyl constituents. Of the remaining 4-1inked residues, half are 2-O-methyl-Dgalactopyranosyl residues and half are galactopyranosyl residues, of which approximately half are in the L configuration.