Abstract: A polarimetric directional field sensor comprises an emitter and receiver (1) of measurement signals, connected by optical fibers (2, 2a, 2b) to a portable probe (3). The probe (3) comprises an optical element (11, 13) sensitive to the field to be measured and means (9, 20) for polarizing the measurement signal. The optically sensitive element (11, 13) is in the form of a birefringent crystal having a crystalline structure which provides at least one plane in which the induced birefringence is modified by a single component of the field to be measured. The crystal is cut and oriented in the probe so that the direction of propagation of the optical measurement signal inside the crystal is normal to this plane, the polarizing means (9, 20) being oriented at 45.degree. to the orthogonal axes in which the crystal's refractive indices are modified by the desired component of the field.