The chemical composition, biosynthesis and ultrastructural organization of the cell wall of cultures of Koliella antarctica (Klebsormidiales, Chlorophyta), a green microalga obtained from samples of sea water taken near the Italian Station of Terra Nova (Ross Sea) during the austral summer 1989–1990, have been studied. Purified cell walls of the microalga were sequentially treated with chemicals which, in higher plants, solubilize matrix polysaccharides (pectins and hemicelluloses) and leave an insoluble residue considered to be α-cellulose. CDTA plus Na2CO3- and KOH-solubilized polysaccharides were made up of glucosyl residues (more than 90 mol%), as well as some minor sugars (mannose and rhamnose). Linkage analysis and enzymic hydrolysis of the matrix polysaccharides indicated the presence of β-1,4-linked glucans with minor amounts of other uncharacterized polysaccharides. The insoluble residue was composed of a small amount of crystalline α-cellulose associated with mannan or glucomannan chains. The biosynthesis of cell wall polysaccharides was studied by incubating the microalgae in the presence of D-[U-14C]glucose or myo-[U-14C]inositol. The radioactive glycosyl residues incorporated into the polysaccharides solubilized from purified cell walls of the cultured microalga corresponded to those detected in the chemical analysis of the cell wall. The myo-inositol oxidation pathway was demonstrated to be functional in the microalga. Cell wall ultrastructural observations showed a loose network of cellulose microfibrils in an extended matrix.