Recently the Northern Adriatic Sea (Italy) has repeatedly been interested by extensive mucilage formation. We have formerly shown that this phenomenon could be related to the abundance of the dinoflagellate Gonyaulax fragilis: monitoring activities evidenced the synchronous presence of mucilaginous masses and that of increasing densities of this microalga; cultured G. fragilis produced abundant polysaccharides, giving rise to a whitish gelatinous material; the monomeric composition of G. fragilis carbohydrates showed an overall good correlation with that of mucilage samples. The present work was performed with the aim of better understanding the nutrient requirement of this alga and thus of linking its proliferation to specific environmental conditions.