This paper describes the isolation and characterization of the polysaccharides synthesized by the unicellular organism, Monodus subterraneus. Complete extraction of polysaccharide material proved difficult and the method finally adopted, after removal of colouring matter by immersion in dimethyl sulphoxide, was successive extractions with (i) cold water, (ii) hot water, (iii) aqueous ethanol, (iv) chlorite solution, and (v) dilute alkali. The major polysaccharide, isolated from (v), is a glucan. It was shown by periodate oxidation, methylation and enzymic studies to consist mainly of β-l, 4-linked glucose units. The presence of a small proportion of β-1, 3-linked glucose units, either incorporated with the β-1, 4-linked units in a single polymer of the lichenin-type or in a separate polymer of the laminarin-type, was also established. Further small quantities of this mixed glucan were also deposited from (ii) and from (iii) on keeping these extracts in the cold. Heterogeneous crude polysaccharide material was isolated from (i), (ii), (iii) and (iv) by alcohol precipitation. The sugars present in these various precipitates and their respective oxopolysaccharides were identified chromatographically, and, in the case of the major constituents, separated and characterized as crystalline derivatives. The polysaccharide extracted by cold water was found to contain glucose, mannose, galactose, xylose and 12 per cent of sulphate; that extracted by hot water comprised glucose (major), galactose (major), and possibly fucose; and that extracted by chlorite solution consisted of glucose (major), xylose (major), ribose and a trace of galactose.