Pelomyxa quarta was re-isolated firstly after its description by A. Gruber in 1884 as Amoeba quarta. This pelomyxa species was found in samples of bottom sediments from the Osinovskoe Lake (North-West of Russia) and investigated using methods of light, immunofluorescent and transmission electron microscopy. P. quarta has a complex of features that are characteristic for representatives of the genus Pelomyxa: amoeboid movement, many immotile flagella, multinuclearity, vacuolized cytoplasm, and presence of prokaryotic endobionts and agglomerations of reserve nutrients in cells. This species clearly differs from the known pelomyxes in the organization of its nuclei, in which a compact nucleolar mass is located eccentrically. © 2024 The Author(s).