A castellated locknut having an annular ring and a set of castellations, wherein the castellations are tapered to provide a locking mechanism. The locknut is provided with a circumferential groove formed between the castellations and the annular body which ensures that the castellations have equal base cross-sectional areas, such that the castellations deflect uniformly upon mounting to a shaft. Further, the provision of the circumferential groove renders the annular ring torsionally flexible as compared to the flexural stiffness of the castellations. Upon mounting to a shaft, the annular ring of the locknut elastically and torsionally deflects, thus allowing the castellations to pivot clear of the shaft without undesirable inelastic deformation. Accordingly, a precise and uniform locknut having an effective locking mechanism is achieved. In an alternative embodiment, the castellations are not tapered; rather the castellations are bent radially by the application of a load prior to mounting to a shaft such that, upon mounting to the shaft, a locking interference is achieved.