Abstract: A cap assembly for sealing engagement with the upper end of a cylindrical steel well casing is provided in two initially separate, upper and lower sections. The lower section has a major opening therethrough for encircling the upper end of the well casing, the diameter of said opening tapering from a dimension at the lower side which is slightly larger than the diameter of the casing, to a slightly smaller diameter at the upper side. The material of the cap and dimensions of the major opening relative to the outside diameter of the well casing are such that the lower section may be manually inserted on the casing, with the latter extending completely through the major opening, whereby the lower section is in tightly sealing engagement with the casing without requiring a gasket. The upper cap section provides a cover for the lower section and includes a recessed portion bounded by a lip of continuous, closed outline extending downwardly from the lower surface of the upper section.