ABSTRACT. – This study is devoted to analyze the fine structure of Cymatotheca weissflogii, Tryblioptychus cocconeiformis, Skeletonema grethae, S. tropicum, Cyclotella atomus, C. choctawhatcheeana and C. litoralis (Thalassiosirales). Net samples were collected from the surface layer of the water column at several coastal stations in Buenos Aires and Río Negro Provinces, Argentina. A new marine planktonic fultoportuloid diatom, Cyclotella cubiculata sp. nov., is described using light and scanning electron microscopy. This species resembles Cyclotella litoralis in general external aspect of the valve except for a minor difference in the position of the opening of the rimoportulae. Nevertheless, Cyclotella cubiculata clearly differs from C. litoralis by presenting marginal chambers easily visible with light microscopy and visible with scanning electron microscopy only in internal view. Comparisons between the new species and some allied brackish and marine species with marginal chambers as Cyclotella striata and C. stylorum are conducted. Cyclotella atomus and C. choctawhatcheeana are recorded for the first time from Argentinian coastal waters, Cymatotheca weissflogii, Tryblioptychus cocconeiformis, Skeletonema tropicum and Cyclotella litoralis are recorded for the first time from Argentina, and Skeletonema grethae is recorded for first time from the South Atlantic Ocean.