Longitudinal area on the ventral surface of dinokont-type cells that forms a pronounced furrow or depression that houses the longitudinal flagellum. In thecated species, the sulcus is made up of sulcal platelets (designated by 's' ). This feature is not present in some desmokont-type cells (Steidinger and Tangen, 1996).

Alternative form for sulcus : sulcal region.