Contractile vacuoles are organelles which occur in many protists and some freshwater sponges. They are defined by their behaviour of slowly filling with fluid and periodically expelling this fluid from the cell (for review see Patterson 1980). Contractile vacuoles form one part of a more extensive organelle, the contractile vacuole complex. The organization of the contractile vacuole complex (CVC) can best be described from ultrastructural studies. The CVC typically includes (1) the contractile vacuole, (2) a system of membranous elements called the spongiome and possibly (3) a permanent pore supported by cytoskeletal structures which ensure the expulsion of the fluid at a fixed site on the surface of the cell. Contractile vacuole complexes have usually been implicated in the osmoregulatory competence of the cells.