The occurrence of iduronic acid in the cell surface glycoprotein of halobacteria is described. This hexuronic acid is not only found in protein-linked oligosaccharides in halobacteria but does also exist in the corresponding dolichol-linked precursors. These findings were unexpected, as iduronic acid is a typical constituent of animal glycosaminoglycans, and its biosynthesis is so far known to occur by epimerization of a glucuronic acid residue within the completed carbohydrate chain linked to protein.