An adapter for a wellhead, the well having a casing and a submersible electric pump mounted within the casing, the casing having an upper end and an upwardly opening bore, the adapter including a base plate having forward and rearward ends, the base plate's rearward end being fitted for mounting over the casing's upper end and having a bore opening aperture therethrough, the base plate's forward end being forwardly oblongated and having a wire passage aperture therethrough; an annular case retaining flange having forward and rearward ends, the annular case retaining flange being fixedly attached to and extending downwardly from the base plate; a switch box having a ceiling, a floor, a left wall, a right wall, and a rear wall, the ceiling incorporating the base plate's forward end and the rear wall incorporating the annular case retaining flange's forward end.