Fabric anisotropy is a key component to understand the behaviour of granular soils. In general, experimental data on fabric anisotropy for real granular soils are very limited, especially in the critical state. In this paper, x-ray tomography measurements are used to provide experimental data on contact fabric anisotropy inside shear bands for two granular soils. The data are then used to assess the validity of Anisotropic Critical State Theory (ACST) and the accuracy of a fabric evolution law that was previously developed from the results of DEM simulations on idealised materials. Overall, the experimental results support ACST according to which unique (i.e., independent of initial conditions) values for fabric anisotropy and coordination number are observed at large strains. With increasing roundness of the material, the rate at which the critical state is approached increases. The evolution of fabric anisotropy measured from the experiments is fairly well reproduced by the proposed evolution law.