Abstract: An improved ball valve is disclosed, which is particularly adapted for use with pipelines of large diameter, possibly of the kind intended to convey toxic or dangerous fluids and for which a virtually perfect seal is required, said valve having a control mechanism which comprises sequentially acting linkages and preloaded resilient means in order that the valve shutter is always in an accurate central position as to its axis of rotation and the approach of the shutter to the seat takes place only once that the rotation of 90.degree. has been completed, in order that a uniform and intense pressural action of the shutter on the seat may be obtained. Alternative embodiments within the scope of the invention are shown and described.