A bag having an inwardly disposed seam is manufactured from a sheet of plastic bag material having two major surfaces and two parallel side edges. A first seam seals the two parallel side edges together at the same major surface with the side edges disposed inwardly of the tube to form the sheet of plastic bag material into a tube with internal flap portions extending interiorly of the tube. At least one seal across the tube is provided for forming at least a three-sided bag structure (and preferably two seals across the tube for forming a closed four-sided bag structure) whereby pressure in the interior of the bag acts against the internal flap portions disposed inwardly of the tube to enhance the sealing of the seam. The bag is filled at an opening defined along the opposite side edges of the seam facing inwardly of the tube.