A valve plug has substantially the same shape as the valve chamber and is rotatably disposed therein. A shaft is movable linearly in response to a signal from a valve actuator. A bellows is sealingly disposed between the valve chamber and the valve actuator and means are located between the bellows and the valve plug for converting linear movement of the shaft connected to the valve actuator to rotational movement of the plug. The invention may also comprise an improved 1/4 turn operator valve including a valve body having an inlet passage and an outlet passage, a valve chamber between the inlet and outlet passages, a valve plug having substantially the same shape as the valve chamber and being rotatably disposed therein, the valve plug having a flow passage there through, the flow passage disposed such that with the valve plug in a first position the flow passage is in alignment with the inlet and the outlet passages and provides fluid communication there between, and with the valve plug in a second position the flow passage is out of alignment with the inlet and outlet passages and the valve plug interrupts fluid communication there between, a shaft connected to a valve actuator movable linearly in response to a signal from the valve actuator, wherein the improvement comprises a cylindrical shaped member sealingly connected by bellow means to the valve chamber, a valve operator shaft passing through the cylindrical member in contact with the shaft responsive to the valve actuator and connected to the valve plug, and mating helical threads o one of the valve operator shaft and the cylindrical member providing rotation to the valve plug upon linear reciprocal movement of the shaft responsive to the valve actuator, whereby a signal from the valve actuator resulting in linear movement of the shaft responsive thereto and rotational movement of the valve operator shaft and the valve plug between the first and second positions thereof.