Abstract: A one piece plastic closure for a container having an externally screw threaded neck, the closure comprising a top portion and an internally threaded skirt. The closure has an annular sealing rib which projects downwardly from the underside of the top portion. The rib includes a first, substantially cylindrical, portion contiguous with the underside of the top portion and lying adjacent or abutting to the skirt and a second, frusto-conical, portion contiguous with the end of the first portion distal to the underside of the top portion and extending radially inwardly to terminate in a circular free edge, such that during threaded engagement of the cap with the neck, the second, frusto-conical, portion will be engaged by a free end of the neck and folded back against the first, substantially cylindrical portion of the rib to form a gas-tight seal between the neck of the container and the closure.