In the plankton of Barn Elms Reservoir (No. 5), Hammersmith, London, Eudorina elegans Ehrenb. was found from November 1945 to January 1946 to be infected by a chytridiaceous fungus bearing^ striking resemblance to Dangeardia mammillata which Schroder (1898) originally described on Pandorina morum Bory. A detailed study was made of the life history of this fungus based entirely on observations from living material. In the parasitized coenobium the chloroplast of each infected cell contracts away from the wall, and is finally reduced to a small mass of yellowish brown granules. As many as thirty individuals of the parasite are sometimes to be seen in a single coenobium, but, provided that one or two cells remain unaffected, the host retains its motility. The story of development of the parasite has been built up by the examination of a large number of individuals in varying stages. The zoospore encysts on the surface of the coenobium and produces a fine germ tube which grows through the mucilage sheath to the nearest host cell. After contact is made with a host cell the germ tube gradually broadens from the proximal towards the distal end, until the thallus is almost cylindrical.