Abstract: A plastic straddling or expansion plug having a closed plug neck and a closed plug foot both with a longitudinal aperture into which a screw can cut a thread. The plug includes a shank having a thin plug jacket on which a two-flight helix is integrally formed. In the region between the helices, the plug jacket is provided with longitudinal slits. On the outside of the helix, further ribs are provided which each extend over a 180.degree. arc and are integrally formed with the helix, alternating in orientation by a 90.degree. angle. The plug neck includes flange lugs oriented in the shape of a cross and adapted to be folded over.