Abstract: A device for the ultrasonic welding of a tubular fitting having an annular flange in an opening of a thin-wall synthetic-resin foil container has an anvil supporting the fitting against the ultrasonic welding head. The anvil includes an upper anvil member and a lower anvil member. The upper anvil member has an upper surface confronting the head and formed in axial alignment with the head with a protuberance holding the foil away from this surface. The surface is provided with an annular groove receiving a first rubber ring registering with the head and supporting the flange of the fitting, the first rubber ring having an elasticity greater than that of the foil of said container. The lower member has a surface juxtaposed with said upper member and provided with an annular recess receiving a second rubber ring. The second rubber ring projects above the surface of said lower member.