An improved pitless well adapter including a supported unit connected to a pump drop pipe and mating with a hanger unit. The hanger unit includes a body portion defining a passageway aligned with a passageway in the supported unit and a nipple portion extending outwardly through an aperture formed in a well casing below the ground surface. A sealing arrangement pressurized by pumped fluid is provided around the casing aperture and, preferably, an anti-syphon check valve is disposed in a seal pressurization passageway leading to the seal arrangement to prevent depressurization of the seal and contamination of the well upon reverse flow of the pumped fluid in the pump drop pipe. Double gaskets in one embodiment from a sealed chamber pressurized by pumped fluid entirely inside of the well casing about the casing aperture.