Abstract: A one-piece plastic cap for the application to the externally threaded neck of a bottle has an end wall with a concave inner side which is provided with at least one elastic annular lip bearing against the end face of the neck when the internal thread or threads of the skirt of the cap mate with the external thread or threads of the neck. The configuration of the lip is such that the sealing action between its annular edge and the end face of the neck increases with increasing pressure in the interior of the bottle. The external surface of the end wall is formed with reinforcing ribs which radiate from the center of the end wall and can also extend along the exterior of the internally concave intermediate portion connecting the marginal portion of the end wall with the skirt. The ribs prevent pronounced expansion of the end wall and/or intermediate portion of the cap in response to establishment of a pronounced pressure differential between the interior and the exterior of the sealed bottle.