Abstract: A self-locking fastener assembly includes a male screw fastener and a related female component (such as a plastic workpiece) for receiving the male screw fastener; a locking element is mounted on, or integrally formed with, the male screw fastener and provides at least one or more flexible fingers extending radially outwardly. The flexible fingers engage the walls of a cavity in the female component. The cavity walls are provided with a plurality of teeth which reliably engage the flexible fingers to prevent counter-rotation of the male screw fastener out of engagement with the female member. In a preferred embodiment, the male screw fastener is a double ended screw having a lower screw end for screwing into a plastic workpiece, and an upper stud end onto which can be mounted a second workpiece, which is secured by a nut.