We report the observation of a novel nonlinear optoacoustic phenomenon, that we name forward stimulated interpolarization scattering. When two frequency-offset laser signals are colaunched into orthogonally polarized guided modes of a birefringent small-core (1.8 mu m diameter) photonic crystal fiber, a pattern of axially moving polarization fringes is produced, with a velocity and spacing that depends on the frequency offset. At values of frequency offset in the few-GHz range, the pattern of moving fringes can perfectly match the phase velocity and axial wavelength (3.9 mm) of the torsional-radial acoustic mode tightly guided in the core. An intense optoacoustic interaction ensues, leading to efficient nonlinear exchange of power from the higher frequency (pump) mode to the orthogonally polarized lower frequency (Stokes) mode. A full-vectorial theory is developed to explain the observations.