We report the experimental observation of a light polarization rotation in vacuum in the presence of a transverse magnetic field. Assuming that data distribution is Gaussian, the average measured rotation is (3.9±0.5)×10-12rad/pass, at 5 T with 44 000 passes through a 1 m long magnet, with λ=1064nm. The relevance of this result in terms of the existence of a light, neutral, spin-zero particle is discussed.