Several species of the chain-forming marine diatom genus Pseudo-nitzschia produce domoic acid (DA), a neuroexcitant amino acid, responsible for a human amnesic syndrome after consumption of toxified shellfish (ASP). Since 1995, the Portuguese monitoring program has regularly detected the presence of DA in shellfish. So far only P. australis has been implicated, and references therein. Identification of the causative species of Pseudo-nitzchia in natural samples is hampered by difficulties in identification at the light microscope level. Recent studies have shown that ultrastructural details of the wall, observed by transmission electron microscopy, allow the identification of species in good agreement with the phylogenetic and biological species concept.