Gyrodinium dominans cells are fusiform from the ventral side, circular or slightly flattened in cross-section, length 23 - 26 microns, width 13 - 15 microns. Hypocone moderately longer than epicone. Both have striations, approximately 8 across the ventral faces. Cingulum deep,with a slight overhang. Distal end approximately 2 - 3 cingulum-widths lower than proximal, displaced approximately 0.3 of the cell length. Sulcus widens as it reaches the antapical end. Longitudinal flagellum arises at the anterior end of the sulcus, in a 2 microns pocket just to the left of the junction of the cingulum and sulcus. Apical groove extends from the cingulum to the apex. Nucleus ellipsoidal, 6-8 microns diameter, in the epicone. Chloroplasts absent.