A vector magnetometer has been built utilizing then = 0, p = 1parametric resonance associated with zero-field level crossings in the optically pumped 23S1level of He4. The principles of operation are discussed and performance is described. The parametric resonance signal-to-noise ratio is 3.5 × 104in a 0.5 Hz noise bandwidth. The linewidth is 9.7 × 10-4G. Techniques for optimizing the sensitivity are discussed, and the sensitivity of the instrument is demonstrated by placing the sensor inside a superconducting magnetic shield and applying calibration signals, The peak-to-peak noise level is 1 × 10-8G. The use of parametric resonance instruments to measure interplanetary and geomagnetic fields is discussed.