This sensor involves the measurement of the amplitude of the Faraday rotation induced by an alternating magnetic field Hm cosQt colinear to the direction of the light beam passing through a thin cell filled with a ferrofluid material (Fe2 Co 04 in dibuthylphtalate or in carbon tetrachiorid) . The peak value of the magnetic field was varied from 0 to 300 gauss, and the frequency f from 10Hz to 40 Khz. Two light beam wavelengths have been tried (1320nm and 1520nm). In AC magnetic fields ( H(t) = Hm cos Qt ) the resultant Faraday rotation is of the form: a(t) = ai cos(Qt+i) a cos(3Qt+3) + a cos(5Qt+45) - . . .etc. We discribe a polarimetric device that gives automaticaly and precisely (0.005°) an angle A of rotation, function of ai , a , a, . . . ,i , Z3 , 'I , ... Accordingly the angle A is only a function of Nm, f, and the ferrofluid cell, then a calibration curve A = f(Hm) can be made. The sensitivity S of this sensor is very high for weak fields (Hm<200 G), obviously depending on the concentration C of the magnetic material (e.g.: C # 5%, A = 1320 nm, S # 180 min/cm.G); a precision of .02 G can be reached. This sensor implements optical polarisaton modulation, thus it is insensible to light beam variations, whether they are due to light source fluctuations, optical misalignment or transmission alteration of some optical component. Furthermore the material is a liquid one, that exhibits no residual birefringence, contrary to most of the solid materials used in usual Faraday modulators. For this reason we think the Fe2 Co 04 ferrofluid material is a good choice for building magnetic fields sensors or polarisation rotators such as those needed in high accuracy polarimetric or ellipsometric devices working on 1320 or 1520nm wavelength. Using ferrofluid materials under longitudinal alternating magnetic field seems more interesting than under the usual dc one [4].