A bag has two juxtaposed face panels of plastic film and each having two parallel side edges and top and bottom edges. A gusset extends between one of the side edges of one of the panels and the juxtaposed side edge of the other of the panels. The gusset is movable between a folded-in condition lying mainly between the panels and an extended condition projecting laterally past the panels and forming a spout generally at the top edges. A slide fastener at the top edges has two profile strips that can be locked to each other and that each have an attachment flap, and a slider shiftable along the profile strips in a closing direction to close the strips together and in an opposite opening direction to separate the strips. Joints secure the attachment flaps directly to inner faces of the respective face panels at the respective upper edges.