Electron-spin-echo-detected electron-paramagnetic-resonance spectroscopy at 95 GHz (W-band) on frozen solutions and single crystals of the azurin mutant M121Q has enabled the determination of accurate principal values of the g-tensor and the orientation of the principal axes of these tensors of all molecules in the unit cell with respect to the crystallographic axes. A combination of EPR and X-ray structural data results in four possible orientations of the g-tensor axes in the copper site of M121Q. Theoretical considerations lead us to prefer one of these, in which one of the principal axes makes an angle of 10 degrees with the Cu-O epsilon(Gln121) bond; the other two are approximately parallel to the NNS plane and are rotated 55 degrees with respect to the Cu-S gamma-Cys112) bond. The description of the rhombicity of the g-tensor indicates that the wave function of the unpaired electron on copper consists of a d(xy) orbital with a small but significant admixture of d(z)2 character.