A stand-up plastic bag has a pair of front and rear side walls defining an open bag mouth at upper edges thereof, and a pair of front and rear bottom gusset walls being disposed between the side walls. The walls are composed of a multi-layered film of different, heat-sealable, coextruded thermoplastics, such as polyethylene and Nylon. Polyethylene is on the interior surfaces and Nylon is on the exterior surfaces of the walls. The gusset walls have upper portions interconnected together to form a bottom wall extending between the front and rear side walls. The gusset walls also have lower portions joined by heat seals to lower portion of the front and rear side walls at contiguous portions of polyethylene material on their interior surfaces to form a support band which connects to and extends below the bottom wall. The front and rear gusset walls also have opposite longitudinal edges joined together by heat seals at contiguous portions of Nylon material on the exterior surfaces thereof which complete formation of the support band as a continuous self-standing structure that defines with the bottom wall a stand-up base on the bag.