In consequence of harmful algal blooms increasing everywhere, also in Adriatic Sea, many regional projects were promoted in the last decade to attend the ecology and oceanography of HAB in order to protect the human welfare. A monitoring program was carried out from August 1996 to November 1997 in the Gulf of Trieste to identify and study potentially toxic algae. Particular regard was devoted to the dinoflagellate genus Alexandrium such as its presence in the seawater as well as toxicity of cultures and blue mussels (Mytilus galloprovincialis) from shellfish farms in the Gulf. Chemical and physical parameters and water samples were collected monthly at four or five depths in four coastal stations in mussel farms. The biological data, based on light and electronic microscopy, revealed the occurence of three Alexandrium species and other 19 taxa of potentially toxic microalgae, but no harmful blooms occured in the studied period. The first presence of toxicity was found from a Alexandrium clone in which gonyautoxins were isolated too. The chemical analyses confirmed the presence of toxicity also in the mussels. The concentration of PSP toxins was always under the limit allowed by the Italian law, but the constant presence of PSP producers in the last years in the Northern Adriatic suggests that HAB and consequent human intoxications could increase in the future. There is still a lack of information regarding the temporal distribution and PSP population dynamics and further scientific investigation is required. An intensive check of the seawater and mussels quality is auspiced in order to safeguard the acquaculture activities and the public health.