The magnetic form factor of uranium in UO2 has been examined by means of neutron diffraction. Uranium dioxide (CaF2 structure) orders antiferromagnetically at 30.8 K with the type I magnetic structure. The intensities of 1250 magnetic Bragg reflections (reducing to 71 independent hkl values and giving full three‐dimensional data for sin ϑ/γ?0.83 Å−1) have been measured with a well‐characterized stoichiometric single crystal at 5°K. The values of (μf) plotted versus sin ϑ/λ show considerable asphericity for sin ϑ/λ≳0.5 Å−1. Comparisons with free‐ion (+ crystal field) calculations give an experimental magnetic moment of 1.78 (4) μB and indicate that the general shape of the form factor is in agreement with theory for low scattering angles, but that the nonspherical contributions at higher angles are at least a factor of five greater than expected. The significance of these results is discussed in terms of the extended nature of the 5f wavefunctions in UO2.