A fixing instrument intended to adjust and lock in adjusted position vertebrae in relation to each other includes a unit comprising two support members which are mutually connected by means of a screw spindle extending through said support members such that the support members are moved towards each other upon rotation of the screw spindle in one direction and away from each other upon rotation in the opposite direction. The support members are pivotally connected to securing members which by means of bone screws are intended to be secured each to a vertebra. The pivot stud has a transverse hole through which extends a locking screw threaded into the support member and having a tapered part by means of which it in active screwed in position by abutment against the wall portion which surrounds the hole in the stud presses the securing member and the support member against each other. Since the bone screw extends through an opening in the support member which is open in the direction facing the support member such that the periphery of the bone screw will be located outside the surface of the securing member a locking of the bone screw is also accomplished when the locking screw is tightened. The bone screw further has a plurality of circumferential grooves and the securing member has a turning bolt provided with a projection which can penetrate into one of said grooves for fixing the bone screw against displacement. All of these lockings thus are carried out from one single position. Instead of the transverse hole (11) the stud may have an annular groove with a tapered limiting surface with which two locking screws may cooperate.