Methylation analysis of a fucose-containing, sulfated polysaccharide (C-II), which was isolated from the brown seaweed Ecklonia kurome and has a potent anticoagulant activity, showed the presence of 3-O- and 3,4-O-disubstituted fucopyranosyl residues in addition to small proportions of nonreducing, terminal fucofuranosyl and fucopyranosyl groups, and 2,3-di-O- and 2,3,4-tri-O-substituted fucopyranosyl and galactopyranosyl residues with various glycosidic linkages. Methanolysis of C-II gave several neutral oligosaccharide fractions in small proportions and two high-molecular-weight acidic fractions in large proportions. Methylation analysis of the low-sulfated acidic fraction showed that the proportion of 3-O-linked fucosyl residues increases and that of 3,4-O-disubstituted decreased as compared to C-II. Methylation and g.l.c.-m.s. analysis of the neutral oligosaccharide fractions showed the presence of Fuc-(1----3)-Fuc and a fucosyl trisaccharide, in addition to small proportions of Gal-(1----4)-Fuc, Fuc-(1----2)-Fuc, Fuc-(1----4)-Fuc, Fuc-(1----2)-Gal, and Fuc----Gal----Fuc. Methylated C-II was also desulfated by methanolysis, followed by remethylation with (2H3)methyl iodide, and most of (2H3)methyl groups were linked to O-4 of the 3-O-linked fucosyl residues. These results suggested a highly branched, new type of fucan sulfate containing a backbone of (1----3)-linked L-fucosyl residues having sulfate groups mainly attached to C-4.