The crystal structure of cyanide-inhibited X. laevis Cu,Zn superoxide dismutase has been studied and refined based on diffraction data collected at 98 K. The final R-factor for the 27,299 reflections in the 10.0-1.7 Å resolution range is 0.170. The cyanide anion, which is a competitive inhibitor expected to mimic the superoxide binding mode, binds directly to the active site copper atom, replacing the coordinated water molecule. Moreover, the anion establishes a strong electrostatic interaction with the guanidinium group of the conserved active site residue Arg 1 4 1 . The coordination sphere of Cu 2 + is partly altered with respect to the uninhibited enzyme: a displacement of 0.41 Å in subunit A, and 0.27 Å in subunit B of the dimeric enzyme is observed for the Cu 2 + ions. Only two ligands in the Cu 2 + coordination sphere (His 4 6 and His 1 1 8 ) are significantly affected by cyanide binding, whereas virtually no rearrangement of the Zn 2 + ligands is reported.