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-l .7 8, 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 Arg14'. The coordination sphere of Cu" is partly altered with respect to the uninhibited enzyme: a displacement of 0.41 8, in subunit A, and 0.27 A in subunit B of the dimeric enzyme is observed for the Cu" ions. Only two ligands in the CL?' coordination sphere (His? and His"*) are significantly affected by cyanide binding, whereas virtually no rearrangement of the Zn" ligands is reported.