We present a high-sensitivity fiber-optic ultrasonic sensor based on a π-phase-shifted fiber Bragg grating (πFBG). A π- phase-shift at the center of a traditional fiber Bragg grating results in the formation of a narrow-bandwidth notch in the reflection spectrum, leading to much higher sensitivity than standard fiber-Bragg-grating (FBG) sensors. A method of fabrication is introduced, including a method of removing introduced polarization dependence. A tunable-cavity diode laser is used to interrogate the πFBG by locking the wavelength to the linear slope of the spectral notch. High-sensitivity detection of ultrasonic waves is demonstrated. The directivity of the grating is characterized. The effect of strain induced polarization dependence is investigated. Preliminary tests using ultrasonic pressure waves in water are performed.