Abstract: A flexible penetration fitting is provided for sealing the hole through which a pipeline penetrates a wall of an underground containment box. The penetration fitting includes a flexible boot with a sealing flange for providing a first seal against the inside wall of the box. The sealing flange is held against the wall by a backing ring placed against the outside wall of the box. The backing ring includes a plurality of studs extending through the wall and into the box. A compression ring with a plurality of holes is placed over the sealing flange with the studs of the backing ring extending though the holes. A plurality of nuts are threaded to the studs to hold the sealing flange firmly against the inside wall of the box. The flexible boot also includes a sleeve for providing a second seal against the outside surface of the pipeline. The sleeve is stepped with different sized openings so as to be able to receive different sizes of pipelines.