Glochidiniumgen. nov., a ncw genus of Peridiniaceae based onPeridinium penardiformeLindemann, is herewith erected. Its plate formula is: Po+X+4′+6′′+3C+4S+5′′′+2′′′′ Main diagnostic characters of this new genus are the presence of only 3 cingular plates (it lacks the transitionalone), the third cingular contacting the anterior sulcal plate, and an unusual sulcus holding a small triangular posterior sulcal plate. The thecal morphology and structure of two freshwater planktic species of the genus are described on the basis of LM and SEM observations.G. penardiformecomb. nov. is an infrequent species, albeit widely distributed world-wide. It has been recorded under the names ofPeridinium, GlenodiniumandPeridiniopsis.Peculiar features in the tabulation of the furrows and of the surface sculpture show that the species does not fit any of the known genera, for which reason the new genusGlochidiniumis established.G. platygastersp. nov., the second species included in the genus, differs from the former by its elongated body, the regular pentagonal shape of its large first apical plate, an equally large sulcal anterior plate, and well developed sculpture, chiefly on the antapical plates.Glochidinium penardiformeandG. platygasterwere found in some reservoirs from central and northern Argentina.G. penardiformewas also found in several Argentine rivers and ponds.