Abstract: Locking device for a steerable castor, comprising a swivel fork connected through a roller bearing to a fixing element which is relatively flat and which essentially takes up little space, the said mechanism comprising an element provided with recesses, which is connected to the swivel fork and turnably connected with the fork, and can be locked in a given position by means of a control lever, characterized in that the locking mechanism is arranged in an essentially flat box-shaped supporting element, which also forms the fastening means of the castor to a trolley, said locking mechanism comprising a relatively large locking plate which can be moved laterally in relation to the turning axis of the fork when the control lever is displaced.