Field-swept electron spin echo spectroscopy at 95 GHz on a frozen solution and a single crystal of azurin from Pseudomonas aeruginosa has enabled the determination of the complete g-tensor. Highly accurate principal values have been obtained (g(x) = 2.0393 +/- 0.0004, g(y) = 2.0568 +/- 0.0007, g(z) = 2.273 +/- 0.004), and the orientation of the principal axes of the g-tensor with respect to the copper site has been established. One of the principal axes makes an angle of 15 degrees with the Cu-S delta(Met121) bond. The other axes lie almost in the NNS plane and are rotated by 24 degrees with respect to the Cu-S gamma(Cys112) bond. The g-tensor does not corroborate descriptions of the copper site in terms of effective C-3v, C-2v, or C-s symmetry. The observed orientation of the principal axes presents a sensitive point of reference for future quantum-chemical considerations.