Spirillum-like bacteria infect Scenedesmus acutus under certain physical conditions of mass culture. They invade the algal cell by perforating its wall, preferentially at one of the two cell tips. As an early event in the development of the disease the host plasmalemma undergoes vesiculation at the site of bacterial infection and vanisches in the other parts of the host cell about at the same time. Beginning in the invaded cell tip the parasites digest the cytoplasm of the Scenedesmus cell and, finally, even the inner, carbohydratous wall layer. Beside this Spirillum-like organism another bacterium sometimes infects and destroys the algae and is, in turn, often lysed by phages.