Azadinium spinosum is a planktonic dinoflagellate, and a source of azaspiracids, a group of lipophilic toxins of human health concern. Little is known about the biology of this genus, and its phylogenetic position within the Dinophyceae is not yet fully resolved. Here, we present detailed observations regarding its vegetativecell division. As a first sign of cell division, the round interphase nucleus enlarges and becomes elongate in a slightly oblique orientation. The nucleus then divides along its longitudinal axis. Cytokinesis of A. spinosum occurs in motile cells and is of the desmoschisis type, i.e., the parental theca is shared between the two sister cells. Divided cells completely separate before the thecal plates are fully replaced. The anterosinistral daughter cell keeps the parent cell ’ s apex, including the apical pore complex and all the apical and epithecal intercalary plates, whereas the posterodextral daughter cell keeps, among others, both antapical plates. In general, the course of the fission line follows the plate overlap pattern and is, with a few exceptions, similar to other species of the Gonyaulacales. A literature survey shows that both desmoschisis and eleuteroschisis have been described for Gonyaulacales and Peridiniales, which suggests that mode of division may not be a good ordinal criterion for classification.