Device in the form of a combined non-return valve and rat killer for internal fitting in a sewer pipe comprising a valve (2), which can swing between an open and a closed position, the valve (2) being able to serve as a seal against possible back-flow (B) and also as a rat killer. The valve (2) is fitted with a projection (2a) on its inner side near its lower end and when swinging down, the blow-impact of the valve with the rat (12) can kill it. The valve (2) is operated by an actuator (7), preferably in the form of a voice-coil linear motor, which is controlled by sensors in the sewer pipe. Due to a continuous flow (A) through the sewer pipe, the device will remain clean and thus functional.