A plastic transmission filter (1) composed of mating upper (2) and lower (3) shells of synthetic resin material with at least one set of registering weld posts (10) extending between inner surfaces of the upper and lower shells; a fluid pervious centertube (4) interposed between the mating shells around the weld posts, and a filter material (5) wrapped around the centertube to form a bag filter. The filter assembly can be welded together in a single vibration welding operation and provides enhanced flow of transmission fluid by avoiding flow restrictions caused by prior art reinforcement poles.