Abstract CeSb exhibits very unusual magnetic properties which can be explained by an important exchange anisotropy, as in some uranium compounds. To better understand the magnetic behaviour of this compound microscopic studies have been undertaken by means of the determination of the magnetization density. Using polarized neutron diffraction, accurate measurements of the magnetic form factor have been carried out. In the paramagnetic region the results show that the crystalline field ground state is the Γ 8 quartet in contradiction with the usual assumption. In the ferromagnetic region the 4f moment is found to be only 1.91 μ B; the reduction of the 4f moment observed in both regions, and the anomalous value of the crystalline field parameter, are discussed in terms of a mixing of 4f electrons with p-electrons of Sb.