  We calculate time correlation functions in the Bak-Sneppen model (Phys. Rev.
Lett. {\bf 71} 4083 (1993)), a model showing self-organised criticality. For a
random neighbour version of the model, analytical results are presented, while
on a one dimensional lattice we give numerical results. The power spectrum of
these correlation functions shows $1/f$- behaviour in both cases.

Condensed Matter