The effect of a high-frequency ultrasonic wave incident normally upon a polarization-maintaining optical fiber is investigated theoretically and experimentally. Two different mechanisms accounting for the change of the birefringence in the fiber due to the radial strain produced by the ultrasonic wave are identified and studied. The more well-known mechanism, which also occurs in a single-mode fiber, arises from the anisotropic strain distribution in the fiber, and is significant only at very high frequencies. The other mechanism, which occurs only in a polarization-maintaining fiber, arises from the inhomogeneous elastic properties of the fiber, and is more significant than the former mechanism at lower frequencies. Because of the latter effect, a polarization-maintaining fiber can respond to lower frequencies than a single-mode fiber in the Polarimetrie mode. The frequency that separates the ranges of significance of these two mechanisms is experimentally found to be about 1 MHz (in water). The principle and the realization of a Polarimetrie ultrasonic sensor using a polarization-maintaining fiber are also described. © 1990, IEEE.