Abstract: A pitless adapter for a well casing is provided with an O-ring seal surrounding the opening in the well casing for the outlet from the pitless adapter to the service pipe. A downwardly facing hook within the area sealed by the O-ring supports the pitless adapter on the lip of the opening in the well casing. A sector shaped locking member on the opposite side of the pitless adapter from the outlet rides in a sector-shaped slot in the pitless adapter and is engaged at the top end of the sector by a screw capable of forcing the sector against the wall of the well casing to force the pitless adapter securely against the wall of the well casing. The locking member is provided with an arcuate channel on its side in which a high friction device such as a rubber pellet is inserted to retain the sector against moderate displacing forces, particularly during insertion of the pitless adapter into the well casing.