Abstract: A locking device arranged on an electrical unit slidably received in a receptacle of a rack, to retain the unit in the rack, said locking device comprising two stop members spaced apart in the direction of insertion of the electrical unit and displaceable relative to one another by means of a manually actuatable drive. The rearmost one of said stop members, considered in the direction of removal of the electrical unit from the rack, being adjustable by means of an adjusting member between an insertion position within the free cross-section of the rack receptacle and a locking position in which a portion thereof extends beyond the free cross-section of the receptacle to engage a rack member of the rack.