The dinophycean family Amphidomataceae encompasses the genera Azadinium and Amphidoma, which attract considerable attention because of their production potential for azaspiracids, i.e. lipophilic polyether phycotoxins. Many new species of Amphidomataceae were described in the last decade, but the diversity probably is not fully explored yet. In a continuation of a previous study, Amphidomatacean diversity of the 1991 spring plankton bloom along the Argentinean shelf was investigated. Based on morphology as inferred from scanning electron microscopy, three new species of Amphidoma and one new Azadinium are described. Azadinium asperum sp. nov. differed in size and shape from other Azadinium and had a characteristically rough and slightly granular surface of thecal plates. Amphidoma trioculata sp. nov. and Amphidoma cyclops sp. nov. differed by shape from most other Amphidoma, and can be differentiated from one another by the presence/absence of a antapical pore and by the size of a ventral depression. Amphidoma alata sp. nov. was unique by a conspicuous wing-like sulcal list. Calculations based on the total number of small dinoflagellates and on relative scanning electron microscopy quantification indicated that density of all four species was fairly high (7–40 × 103 cells L−1), so a large number of cells could be investigated in the sample. A few more single cells in the sample are attributable to Amphidomataceae but do not belong to one of the described species. This indicates that the diversity of the group is still not fully explored.