Abstract: An improved bag construction, and a method of bag forming, are disclosed which gives an economical yet strong, burst- and leak-resistant closure without the necessity of sewing or use of expensive stepped cut tubes. The bag is formed using a tube having opposed, interconnected front and rear panels, wherein a pair of laterally spaced, parallel cut lines are provided adjacent the top margin of the tube which define opposed first and second marginal flaps respectively in the front and rear panels. In bag-forming operations, the first flap is folded and glued against the outer face of the front panel, and the closure is completed by folding and gluing the second flap over the open end of the tube in a manner to refold the first flap and engage the outer face of the first panel. In another embodiment, a filling valve is inserted between the front and rear panels adjacent the closure end during the bag-forming operation, so as to provide a valve bag.