A locknut system having a projecting screw threaded member with a first external thread. A nut is mountable on the projecting screw threaded member by means of a corresponding thread. A second different screw thread is formed on the projecting screw threaded member. The second screw thread is of a different hand or a different pitch from the first screw thread. A locking member has a screw thread for engagement with the second screw thread of the projecting screw threaded member and has an engaging surface which can be brought into abutting contact with a corresponding engaging surface on the nut by mating the thread of the locking member with the second thread of the projecting screw threaded member. A coupling member operates to couple the locking member and the nut to substantially prevent relative rotation there between. One of the locking member and the coupling member carries a snap ring which snaps into engagement with a retaining recess provided on the other when the coupling member is mounted in its use configuration, to prevent accidental displacement of the coupling member from the locking member.