Two Michelson interferometric fiber-optic hydrophones that use panda polarization-maintaining fibers and devices have been constructed and tested. The low-frequency acoustic sensitivities of both hydrophones are $-159\mathrm{dB}$ re $1\mathrm{rad}∕\mu \mathrm{Pa}$ . One of the hydrophones tested has a small cylindrical Helmholtz resonator that has a break point near $1200\mathrm{Hz}$ and a measured roll-off of approximately $50\mathrm{dB}∕\text{octave}$ , and it is a hydrostatic-pressure-insensitive design. This hydrophone is a prototype device for a class of sensors that can be used to eliminate aliasing in future sonar systems. To our knowledge, this is the first time that such a fiber-optic hydrophone has been reported.