The structure of the agarlike sulfated galactan from the epiphytic red alga Laurencia coronopus was elucidated by partial reductive hydrolysis. In the presence of borane-4-methylmorpholine complex, the acid-labile glycosidic linkages of 3,6-anhydrogalactose residues of the polysaccharide are hydrolyzed with the simultaneous reduction of these residues, thus precluding their further acidic degradation. The main products of this hydrolysis were 3,6-anhydro-4-O-beta-D-galáctopyranosyl-L-dulcitol (agarobiitol), its 6-O-methyl derivative, agarobiitol 6-sulfate, and some higher oligosaccharides. These products were structurally analyzed by composition, mass spectrometry, and NMR.