A two-way ball valve (10) for liquid and/or gas flowing media, essentially comprises a valve fitting (12) made up of two connected housing parts (22, 28), in which a ball (18) with a drilling (20) for the media flow and an operating shaft (68) running perpendicular to the drilling (20) is mounted such as to be able to rotate. Furthermore, sealing shells (40, 42) are arranged to both sides of the ball (18) respectively, coaxial to the longitudinal axis (L) of the valve fitting (12), which enclose the ball (18) and completely fill the dead volume (38) between the valve fitting (12) and the ball (18). Said sealing half-shells (40, 42) have a front opening for the operating shaft (38) in the axial direction (L), corresponding to the cross section of the drilling (20) in the ball (18). The sealing half-shells (40, 42) only contact the ball (18) in the region of the front openings thereof, between which a narrow sickle-shaped annular gap is formed. In one embodiment a control diaphragm (58) is include on the inlet side of the flowing medium (16).