Abstract: A gas sampling apparatus including a flexible, substantially gas-impervious, closed bag and a valve assembly mounted in a wall of the bag for providing control of gas flow between the interior and exterior of the bag. The valve assembly includes a valve body mounted in the bag wall for providing a first gas conduit to and from said interior of the bag, and a valve stem for providing a second gas conduit, the stem terminating at one end in a ball mounted on the valve body. The stem is movable about the ball between an opened position in which the gas conduits are in communication with one another and the bag may be filled or emptied through the coupled conduits, and a closed position in which the ball seals the first conduit. A valve cap is provided for capturing the stem at variable pressures between the valve body and the cap. The cap has a slot in one side in which the stem is movable tranversely to its long axis between its opened and closed positions.