The sulfated glycans in the tunic of Styela plicata differ from the glycosaminoglycans of animal tissues and also from the sulfated polysaccharides isolated from marine algae. The ascidian glycans occur primarily as three fractions that differ markedly in molecular weight and chemical composition. The high molecular weight fraction encompasses a broad range of molecular weights but is chemically homogeneous and contains an unusual amount of galactose. The 20,000 molecular weight polysaccharide is rich in galactose and glucose while the 8,000 molecular weight fraction is rich in amino sugars and contains the neutral hexoses galactose, glucose, and mannose. All fractions contain large amounts of sulfate esters. The ascidians polysaccharides can be extracted from the tissue by proteolytic enzyme or by guanidine hydrochloride solutions. The high molecular weight fraction is preferentially extracted by papain while guanidine hydrochloride removes mainly the low molecular weight polysaccharides. We speculate that these sulfated glycans are essential for maintaining the structural integrity of the tunic, in analogy with the glycosaminoglycans of vertebrate connective tissues.