A linerless closure of one-piece integrally molded plastic construction includes a base wall and a cylindrical skirt for attaching the closure to a container neck finish. First and second circumferentially continuous annular walls extend from the base wall at positions radially inward from the skirt. The first and second annular walls are spaced from each other to receive an end portion of the container neck finish when the closure is applied to the neck finish. At least one of the annular walls has a circumferentially continuous annular projection extending toward the opposing annular wall. The annular projection is axially flexible for bending and folding toward the base wall and toward the at least one annular wall in sealing engagement with the end portion of the container neck finish when such end portion is received between the annular walls.