Experimental and theoretical results are presented for the Brillouin spectrum of single‐mode optical fibers in the 180° scattering geometry. It is shown that for core diameters less than 10 &mgr;m both the Brillouin frequency shift and spontaneous linewidth are influenced by the nature of the normal acoustic‐mode spectrum and wave‐vector nonconservation in the acousto‐optic interaction. The velocity and lifetime of longitudinal phonon excitations in the core are evaluated, and the consequences to stimulated Brillouin scattering assessed.