of the sidewalls 30 32 of the retaining grooves 28 in this case basically corresponds to the contour which is predetermined by the two sidewalls 20 22 of the blade root 16 as a result of this the rotor blade can be inserted by its blade root 16 into the retaining groove 28 in a form fitting manner the sidewalls 30 32 of the retaining grooves 28 are correspondingly curved like the sidewalls 20 22 of the blade root 16 the direction of curvature in the circumferential direction in this case is selected so that an essentially convexly curved sidewall 30 of the retaining groove 28 in the rotational direction r of the rotor or of the rotor disk 25 is trailing in relation to the sidewall 32 of the corresponding retaining groove 28 lying opposite it a rotor blade 10 which is inserted in the rotor disk 25 bears essentially against the surface regions which are identified by a a u2032 and b b u2032 during rotation of the rotor these surface regions form the bearing flanks a a u2032 and b b u2032 of the rotor blade 10 and of the retaining groove 28 so that the rotor blade 10 is secured against radial movements on account of the matching arrangement of the curvature of the blade root 16 and the rotational direction r of the rotor an increase of the stress loading in the regions a a u2032 b b u2032 can be avoided of particular advantage in this case is that on both sides of the blade root center line 24 the centers of the respective bearing flanks a b lie upon different radii r 1 r 2 as a result the effect is achieved of the distance between the two centers being greater than.