A stopper made of molded synthetic material comprises a domeshaped head and a hollow cylindrical body depending therefrom, the said body being closed at its lower end. The periphery of the body is provided with annular protruding ribs which coact with the inside of the bottle neck to insure fluid-tightness and these ribs are of substantially sinusoidal shape, each comprising an even number of undulations or wave lengths. With such an arrangement, if the neck has a zone of somewhat restricted diameter and/or of somewhat non-circular cross-section, as such is often the case with bottles for sparkling beverages, the stopper may nevertheless be easily extracted from the bottle by rotating it within the neck while it is submitted to a pulling force.