A method and a device for detection and signalling the presence of preys, which can be fixed to a trap (1) comprising a box body (2) with inlet ports (3) and a over (4). The inlet ports 3 are equipped with a one way inlet mechanism (6,7,8). An animal that enters the trap 12, once passed a central point, causes the movement of the mechanism (6,7,8), which once passed returns back to a starting position. For signalling the presence of vermin in the trap this mechanism is used, for example by means of a push button (15) operated by the their movement. Alternatively, the means for detecting provide the use of one or a plurality of elongated elements (18) operated by the movement of the vermin in the trap. Such elements can be arranged both vertically and horizontally. A further embodiment provides a photocell or a sensor of presence arranged in the block of the trap. The means for signalling can comprise both a light device (21) and a sound device (22), arranged either on the trap or on a remote support (23). The connection can be carried out with a electrical cable (24) with a transmitter (26) and a remote receiver (25), connected to the detecting device. The means for signalling can communicate the entry of the preys in the trap with a predetermined delay time, so that other vermin that can be present enter the trap before the intervention of the operator. The device, according to the present invention can be used for monitoring several traps at the same time.