The response of an optical fiber in water to low‐frequency acoustic waves was investigated experimentally and compared with analytical results. A single‐mode optical fiber was used to pass a laser beam through a tank in which an approximately plane acoustic wave was produced. A change in the optical index of refraction of the fiber creates an effective path‐length change for the optical beam which results in a phase shift of the optical beam with respect to a reference beam unaltered by the acoustic field. By mixing the phase‐shifted beam with a reference beam of constant phase in a photodetector the phase variation at the acoustic frequency is detected. Theoretically, the sensitivity of a fiber‐optic interferometer should be independent of frequency. Experimental results confirm this.