The contractile vacuole of the chloromonadophycean alga Vacuolaria virescens is a permanent structure that possesses a specialized membrane: subunits of this membrane have a diameter of 21-24 nm and in places are arranged in a regular hexagonal pattern. The lateral walls of these subunits form regularly spaced bristles or pegs which extend inwards from the trilaminar membrane for a distance of 13-15 nm. The contractile vacuole is situated immediately above an extensive Golgi apparatus that covers most of the anterior surface of the nucleus. Vesicles of Golgi origin give rise to subsidiary vacuoles which in turn empty into the contractile vacuole. Golgi vesicles, subsidiary vacuoles and the contractile vacuole contain similar electron-dense material. It is suggested that this material might be a highly hydrophilic substance which will attract water from the cytoplasm into the Golgi vesicles, subsidiary vacuoles and contractile vacuole from whence it is discharged from the cell. This method of osmoregulation is compared to that occurring in other algae and protozoa.