Abstract: A closure of flexible molded plastic material is adapted to seal around the mouth at the open end of a storage container and comprises a main closure wall having an outer periphery and an inside surface adapted to span across and close the container mouth when inserted into the open end. The main closure wall includes a central portion deflectable inwardly toward an opposite end of the container when initiating a seal across the open end thereof. A peripheral flange is formed around the main closure wall to extend transversely away from the inside surface and includes a deflectable depending skirt having a free end portion spaced outwardly of an outer peripheral edge and the inside surface of the main closure wall separated therefrom by an annular groove.