Abstract: A plastic fluid tight container and closure assembly comprising a plastic container having a base, a sidewall extending upwardly from the base terminating in a rim defining an opening for the container, said sidewall defining on its inside a sloping abutment surface extending inwardly and downwardly from said rim and an annular notch terminating said abutment surface, and a plastic closure having a circular cover portion, a U-shaped annular rim joined to the periphery of said cover portion, said U-shaped rim having an inner wall and a radially spaced outer wall connected by a base to said inner wall, the outside of said outer wall defining an annular shoulder positioned above the level of the base, an abutment surface extending upwardly and outwardly from the shoulder and an angular annular flange joined to the upper end of the outer wall to form an annular groove, said closure when assembled on said container having its shoulder locked to the notch of said container, its abutment surface in liquid sealing co