Abstract: A rotary fiber optic waveguide coupling. Bared ends of two fiber optic waveguide cable sections are held in mutually-aligned end-to-end adjacent positions in opposite extension along a common rotary axis by a pair of ferrules having central through-openings in which such ends coextend and are bonded in close fit. The ferrules are held in precise axial position by abutment at their inner ends with annular shoulder in a pair of alignment caps mounted in two members movable relatively about such rotary axis. Retaining sleeves screwed into the relatively turnable members cooperate with washer members on the ferrules to obtain such abutment, and the ferrules project through openings in the alignment caps to obtain precise radial positioning. A friction-free rotary bearing interconnects the two relatively turnable members. The bearing is preloaded along the axis of rotation to give precise axial and radial positions of the one turnable member relative to the other.