The atomic dynamics of the enzyme Cu, Zn superoxide dismutase has been investigated by means of quasi-elastic neutron scattering in the temperature range 4-320 K and in an extended momentum range up to 8.5 Angstrom(-1). Below 200 K the integrated elastic scattering intensity can be well described by a quasi-harmonic model, while above this temperature a pronounced decrease of the elastic intensity is observed, together with the onset of a small quasi-elastic component. This behaviour, which is similar to that already observed in other globular proteins, can be attributed to the onset of torsional degrees of freedom which give rise to transitions between slightly different conformational substates of the protein tertiary structure.