The desmid Penium margaritaceum is a common resident of biofilms of shallow Adirondack wetlands in New York State, USA. It was isolated and grown in the laboratory where it readily formed biofilms and produced large amounts of extracellular polymeric substance (EPS). The EPS was separated into two fractions: an EPS gel and soluble EPS. Both fractions were rich in xylose, fucose and glucuronic acid. The EPS gels contained large amounts of 3-linked, 4-linked and 3,4-linked fucose, 3,4-linked glucuronic acid and terminal xylose linkages. The EPS gel consisted of a fibrillar matrix that linked cells and cell substrate together. Immunofluorescence analysis using an anti-EPS antibody revealed that EPS secretion occurs in several different modes, which contributes to initial adhesion, capsule formation and gliding.