Dynamic light scattering measurements on bovine serum albumin (BSA) at pH=4.7 and ionic strength I≊0.12 M were carried out in the pressure interval 1-4000 atm and for concentrations ranging from 8-80 kg m-3. At moderate pressures the mutual diffusion coefficient D was found to decrease linearly with concentration, while at the highest pressures deviation from linearity was observed. For a given concentration, the value of D increases initially with pressure and passes gradually through a maximum; at higher pressures (above 1000 atm) D decreases with increasing pressure. The hydrodynamic radius, deduced from diffusion data extrapolated to infinite dilution, was observed to increase with pressure during the denaturation process (pressures above 1000 atm) of the protein. Upon releasing the pressure the return of D was not reversible.