A spinning rotor for use at high speeds in open-end spinning machines has a rotor cup having an annular wall portion defining a rotor opening, a bottom wall portion merging with the annular wall portion, and a mounting hub extending exteriorly from the bottom wall portion. A rotor shaft is affixed to the mounting hub coaxially with the rotor cup. The merger of the bottom and annular wall portions of the rotor cup defines a maximum exterior diameter of the rotor and an interior rotor groove. The annular wall portion, the bottom wall portion and the mounting hub are of different cross-sectional thicknesses with the cross-sectional thickness of the bottom wall portion increasing from the rotor groove to the mounting hub. The wall thickness is less than 1.2 mm in the area of the maximum exterior diameter of the rotor cup (2) and the rotor cup has an aerodynamic exterior contour free of edges for deterring turbulent boundary layer air flow thereabout and an interior contour for promoting fiber spinning into a yarn.