Abstract: A loss-proof fastener of the type comprising a fastener member including a shank portion having an annular groove (2) and a ring (3) in the groove and transversely split (7) so as to provide an elastic deformation in the direction of the circumference, whereby in a rest condition the ring projects beyond the shank portion surface and in a compressed condition is retracted within the groove, the ring having a cross-section such that the ring is elastically deformable so as to be capable of transverse elongation and radial contraction when the ring is compressed, and the ring when positioned in the groove having a lateral play to permit the cross-section thereof to freely deform when compressed between the rest and the compressed conditions.