Stimulated Brillouin scattering of a laser beam from a Bose-Einstein condensate is calculated and it is shown that the threshold power density of phonon generation is anomalous small, about mW/cm 2 in comparison with a typical value about 10 1 0 W/cm 2 in solids. It is shown that the threshold power density is inversely proportional to the scattering length of a BEC.