Abstract: A locking device with at least one key and a locking cylinder having at least one cylinder core with a key channel. The core is mounted rotatably in a cylinder housing, mechanical tumblers for blocking and releasing the cylinder. An actuator is disposed in the cylinder housing for shifting a blocking element blocking and releasing the cylinder core. An electronic code transmitter is provided in the key and an electronic code evaluator in the cylinder housing for controlling the actuator. The electronic code transmitter is formed by a transponder (26) and the electronic code evaluator by a transponder reading device (16, 17). A power supply device (24) is disposed in the cylinder housing (1). The blocking element (23) engages in at least one recess (42, 43) disposed on the periphery of the cylinder core (4) spaced from the key channel (10, 11) in the axial area of the cylinder core (4) which is provided with the mechanical tumblers (49, 50, 51).