The behavior of the contractile vacuole of Tetrahymena pyriformis W has been recorded and analyzed quantitatively by cinephotography. The vacuole fills in a stepwise fashion by the confluence of ampullae which appear regularly at the beginning of systole and whose membranes are continuous with that of the contractile vacuole throughout the cycle. The vacuole may subsequently fill slowly by a means not discernible by light microscopy. The vacuole rounds up at the beginning of systole and shortly thereafter the ampullae reappear around the perimeter of the vacuole. They are expanded by fluid forced into them from the vacuole. Round-up and the mode of growth of the ampullae indicate that the contractile vacuole is truly contractile. Expulsion occurs soon after the appearance of the ampullae and terminates the cycle. Contraction is initiated at regular intervals by a timing mechanism which is independent of the size of the vacuole. Suitable terminology to describe the structure and behavior of the contractile vacuole is discussed.