We show that in optical fiber the threshold exponential gain, G th, for stimulated Brillouin scattering initiated by spontaneous Brillouin scattering is functionally and strongly dependent on the material, length and numerical aperture of the fiber and the pump wavelength. For silica fiber we show that the value of Gth, at ?? 1 µm ranges from as low as ~5 in long fiber (= few kms) to ~10-12 in fibers of ~ 100 m length and ~20-23 for very short fibers (<10cm). © 2007 Optical Society of America.