Thalassomyxa australis is a plasmodial rhizopod of the sea performing an obligatory change of phases. In the motile phase it displays the appearance of a reticulate plasmodium which creeps about and ingests the food by phagocytosis. Besides, plasmotomy and fusion with other plasmodia may occur. In the resting phase no visible movements are discernible. The plasmodia are characterized by their large digestive vacuoles. The film shows transitions between both phases, especially from the resting to the motile phase. The last scene which took a whole week, illustrates the grazing of a "diatom lawn" with repeated, synchronous changes of phases.