The behaviour of contractile vacuole complexes of representatives of the Cryptophycean genera Chilomonas, Cryptomonas, Cyathomonas and Rhodomonas is described using light and electron microscopy. The contractile vacuole occupies a fixed position in the cell, lying adjacent to the flagellar pocket/groove. A small region of the flagellar pocket is invaginated so that the plasma membrane lies close to the membrane of the contractile vacuole. This region acts as the pore through which the contents of the contractile vacuole are discharged. Discharge follows the fusion of the contractile vacuole membrane with the plasma membrane of the pore. Filling initially involves the fusion of small vacuoles which appear around the contractile vacuole shortly before the discharge of its contents. After these vacuoles have fused, filling may continue by the accretion of small vesicles which form the spongiome surrounding the contractile vacuole. Two types of vesicles have been identified in the spongiome. One type is invol...