TmSe is an intermediate valence compound. Below TN = 3.2 K it orders antiferromagnetically and becomes an insulator. This order is destroyed by a magnetic field larger than 3.5 kOe: it becomes a conductor and behaves ferromagnetically. We have measured the Tm magnetic form factor in its different states. In the paramagnetic state and in the induced ferromagnetic state the form factor corresponds to 4f electrons. In the antiferromagnetic state the experimental form factor deviates at low angles from both Tm2+ and Tm3+ form factors indicating a polarisation of the outer electrons. The direction of this polarisation, in contrast to what happens for the second rare earth series, is opposite to the magnetization of the 4f electrons. Tm apears to be equivalent of Ce in the first half. © 1986.