A structural study of the cell wall polysaccharides of Myrmecia biatorellae, the symbiotic algal partner of the lichenized fungus Lobaria linita was carried out. It produced a cold-water insoluble rhamnogalactofuranan, with a (1→3)-linked β-d-galactofuranosyl main-chain, substituted at O-6 by single units of β-d-Galf, or by side-chains of 2-O- and 2,4-di-O-linked α-l-Rhap units. The structure of the polysaccharide was established by chemical and NMR spectroscopic analysis.