Abstract: A ball valve construction particularly suited for forming large diameter ball valves includes a main valve body formed from extruded plastic pipe and having a central through passage with a valve ball mounted centrally therein. An operating stem extends from the ball outwardly of the main body. First and second separate seat retainer rings are positioned on opposite sides of the valve ball. Each retainer ring has an outer surface carrying a seal for engaging the wall of the through passage for preventing fluid flow between the retainer rings and the wall of the through passage. Each retainer ring further defines a spherical seat surface positioned and sized to sealingly engage the valve ball. Adjustable tie rod members join between the retainer rings to cause the seat surfaces to engage the valve ball with a predetermined sealing force.