Cells of the potentially toxic dinoflagellate Alexandrium catenella have a complement of features typical of dinoflagellates, including dinokaryon, pusule and amphiesma. No endosymbiotic bacteria were found in the Californian strain examined, which is weakly toxic. The chloroplast consists of a central pyrenoid complex with radiating chloroplast lobes, an arrangement found also in certain other gonyaulacoid dinoflagellates. Cells are often arranged in few-celled chains and a cytoplasmic connection interconnects adjoining cells. Striated fibres line the attachment canal and also subtend the apical pore, making up an ‘apical fibrous complex’. The flagellar apparatus basically resembles that of other gonyaulacoids. However, Alexandrium catenella, Gonyaulax spinifera and Protoceratium reticulatum share the presence of two striated collar connectives and a single-stranded microtubular root associated with the longitudinal basal body, in addition to the multi-stranded root characteristic of dinoflagellates in general. Ultrastructural features indicate that these three species are closely related while Ceratium furcoides, another gonyaulacoid dinoflagellate, apparently lacks these features.