Monoclinic crystals of native bovine superoxide dismutase and its monocyano derivative were studied by means of electron paramagnetic resonance spectroscopy. Through computer simulation of the spectra, the directions of the principal axes of the magnetic tensors (g and A) have been found with respect to the crystal principal axes and with respect to the positions of atoms bear the Cu(II) as previously determined by x-ray crystallography (Richardson, J. S., Thomas, K. A., and Richardson, D. C. (1975) Biochem. Biophys. Res. Commun. 63, 986-992; Tainer, J. A., Getzoff, E. D., Richardson, J. S., and Richardson, D. C. (1980) in 2SOD: Cu, Zn-Superoxide Dismutase Complete Atomic Coordinates (Richardson, D. C., and Richardson, J. S., eds) Brookhaven Protein Structure Data Bank). In the native protein, the direction of the gz axis of Cu(II) was found to lie perpendicular to the rough plane formed by the four imidazole nitrogen atoms coordinated to the Cu(II). The direction of gy is approximately along the His 44N-Cu-His 46N direction, and gx is in the direction of the Cu-His 61-Cu-N bond. The A is coaxial with g within 15 degrees C. A substantial shift occurs in the direction of gz when CN- binds to the Cu(II), suggesting a change in the coordination configuration of the metal.