The genus Fallacia Stickle & Mann includes naviculoid birraphid diatoms with H-like plastids, valves with uniseriate striae and covered by porous conopea, rounded areolae occluded by hymens, and depressed hyaline lyriform area characterizing the valvar surface. The species Fallacia marnieri was found in samples collected in King George Island, Antarctic Peninsula, on which an electron microscopy study and literature revision were carried out. From this investigation, we found that since Fallacia marnieri has the diagnostic structures of the genus, its recent inclusion in Fallacia is justified. The species was found on rocks of an intertidal region, and seems to be endemic in cold regions, as indicated in the literature.