Abstract: A tamperproof rotary valve which includes a valve body having a fluid flow passage extending therethrough. The passage is intersected by an actuator opening in the body and defining a valve chamber thereat. A rotary valve is disposed in the chamber for opening and closing the fluid flow passage. An actuator is journaled in the actuator opening and is coupled to the rotary valve for rotating the valve. A groove is formed about the actuator, and a contractible/expandable locking ring is disposed in the groove and projects outwardly of the groove beyond the bounds of the actuator when in a free, expanded condition. The locking ring is contractible into the groove to allow assembly of the actuator into the actuator opening. A locking groove is formed in the actuator opening in the valve body and into which the locking ring expands when the actuator is in a proper assembled position in the actuator opening.