Abstract: A retaining ring structure is adapted to be located in a circumferential groove in a first member and to engage a second member to restrain relative axial movement of said members under thrust loading. The retaining ring structure includes a ring portion of L-shaped cross section having a leg portion and a foot portion. The free end of the leg portion fits into the groove and the foot portion engages a surface of the member adjacent the groove. A surface of the leg portion is adapted to contact the sidewall of the groove and is undercut so as to avoid contact with the edge of the groove. One specific retaining ring structure includes a pair of sector ring portions in nested relation with the outer apex of the L-shape of one of the sector ring portions located at the inner apex of the L-shape of the other sector ring portion.