The species composition and structure (e.g. abundance and biomass) of protistan plankton (cell size > 5 µm) and in situ chorophyll a were assessed in a shallow (<50 m depth) inner shelf area of the Argentine Shelf called El Rincón (38º-41°S). Surface water samples (5 m depth) for plankton quantification) were taken with Niskin bottles during four oceanographic cruises (two in early austral spring and two in late austral summer- early fall), onboard the vessel B. Houssay accounting for a total of 36 sampling stations. These samples were analyzed under optical microscopy following the inverted microscope technique with sedimentation chambers. Cells enumeration and identification was made up to species, genus or family level, which were afterward categorized in taxonomical groups: diatoms, dinoflagellates, coccolithophores and nanoflagellates. The studied area supports important fishes of commercial interest, therefore plankton biodiversity records are neccesary to understand possible shifts at the population and community levels that might have cascading effects on marine ecosystems' productivity.