Abstract: The invention relates to an anti-theft device for cycles comprising one or more frame tubes and a pedal crank housing attached thereto, characterised in that a through drilling is arranged in the middle of the pedal crankshaft as well as of the fixing screws. In the mid-region of the pedal crankshaft is a recess in which a lock housing with a cylinder lock is arranged. The key of the cylinder lock axially extends through the fixing screw closest to the cylinder lock. On the opposing front face of the cylinder lock an eccentric pin is arranged which co-operates with the front face of a slider displaceably arranged in the recess, by means of an eccentric pin guide. In the front face of the slider, opposite to the eccentric pin guide, a locking groove with a central drilling is arranged, into which the locking head of a lock pin, arranged in the through drilling of the second fixing screw and the pedal crankshaft extends.