Abstract: A container having an external screw thread is closed by a synthetic plastics stopper comprising an internally threaded cap with a seal part protruding from the cap top, this seal part having an outwardly protruding seal section which in use is bent back outwardly by the inner wall of the container neck. The screw thread on the container neck, the screw thread on the cap and the seal section are such in relation to one another that in applying the stopper the seal section contacts the container neck and is bent back thereby before the external screw thread on the container and the internal thread on the cap come into engagement with one another. The seal section preferably consists of permanently deformable material so that on unscrewing the stopper, the seal between the stopper and the container opening is broken before the threads come out of engagement.