The green Met121His mutant of the blue-copper protein azurin has been investigated by pulsed electron paramagnetic resonance (EPR) spectroscopy at 95 GHz on a single crystal. The axial histidine is more strongly bound to copper than the methionine for the wild-type protein. The g tensor of M121H is found to be virtually axial and the z principal axis perpendicular to the plane spanned by copper and the nitrogens of the ligating histidines 46 and 117. The direction of the x axis is close to the bond direction from copper to the nitrogen of histidine 46. Theoretical analysis of the axiality and the orientation of the principal axes shows that the wave function of the unpaired electron on copper, largely d(xy) for blue-copper proteins, acquires some d(yz) character for M121H. Comparison of these results with data for wild-type azurin and the mutant M121Q provides insight into the subtle relation between the electronic and the geometric structure of blue-copper sites.