Polysaccharide-specific staining techniques have revealed the existence and high abundance of gel-like mucilaginous particles formed from the polysaccharide exudates of diatoms. Evidence from both the field and laboratory indicates that these transparent exopolymer particles, known as TEP, are a major component of diatom aggregates and facilitate the flocculation of diatom blooms. TEP are composed primarily of sulfated polysaccharides, molecules known to be resistant to bacterial decomposition. Furthermore, the presence of TEP in diatom aggregates reduces the permeability of aggregates causing them to become neutrally buoyant and accumulate at picnoclines where they eventually develop gas bubbles and float to the surface. Diatom species, such as Cylindrotheca closterium, found abundantly in the Adriatic Sea, may produce particularly unique and copious quantities of TEP resistant to sinking and degradation. This material could accumulate as mucilaginous nuisance scums when environmental conditions favor strong stratification of the water column, high diatom biomass, and the dominance of these species.