Forward light scattering by the thermally excited guided acoustic modes of an optical fiber produce numerous narrow lines not predicted by the usual theory of Brillouin scattering. Optical heterodyne detection has been used to resolve the scattering spectrum which begins at about 20 MHz and extends to the detection limit. A simple theory quantitatively accounts for the frequencies, polarizations, and intensities of the components. The light scattering from these modes constitutes a thermal-noise source in optical fibers that may prove significant in other experiments.