Dinophysis sacculus (Dinophyceae) is a potential producer of diarrhoeic shellfish poisoning (DSP). Population dynamics of this dinoflagellate was followed in the Varano lagoon, south-east Italy, with the objective of determining the seasonal abundance of D. sacculus and the relationship to environmental factors. The seasonal occurrence of this dinoflagellate showed a rather wide temporal distribution and increases in cell numbers were observed in late June and November. Dinophysis numbers seasonal changes appeared to be dependent on chemico–physical factors and dissolved nutrients. Three morphologically distinct forms of D. sacculus were found during the sampling period (March 1997–February 1998) related to the life history of the species and not to environmental conditions. In this mussel fishery, D. sacculus presence and proliferation were not associated with biotoxicity.