The dispersion properties of ion acoustic waves and their nonlinear coupling to light waves through ponderomotive and thermal forces are sensitive to the strength of electron-ion collisions. Here, with the neglect of electron-electron collisions, the growth rate of stimulated Brillouin scattering (SBS) is obtained when the driven acoustic wave frequency and wavelength span the range of small to large compared to electron-ion collision frequency and mean free path, respectively. In all cases the thermal contributions to the SBS growth rate are insignificant if the ion acoustic wave frequency is greater than the electron-ion collision frequency and the wavelength is much shorter than the electron-ion mean free path. On the other hand, the purely growing filamentation instability remains thermally driven for shorter wavelengths than SBS even when the growth rate is larger than the acoustic frequency role. (C) 2005 American Institute of Physics.