Abstract: A releasable locking device for mounting in the bore of a shaft or the like for retaining an element thereon. The releasable locking device is in the form of a cotter ring comprising a one piece rod formed of flexible material and having a leading portion, a central portion and a trailing portion. The central portion is wound in the form of a helix so as to form more than one but less that two full helical turns with adjacent turns being separated by a distance insufficient for the rod to be freely trained through the bore of the shaft during mounting. The leading portion extends tangentially from the central portion so as to define a lead that is freely insertable into the bore, and the trailing portion is helically shaped in axially flared relation to the central portion in a manner that progressively increases the axial spacing between the trailing portion and the immediately adjacent turn of the central portion to facilitate removal of the device from a locked position.