The basal-plane anisotropy of the antiferromagnetic phase of the hexagonal heavy-fermion superconductor has been studied via the magnetic torque. Torque measurements were performed as functions of magnetic field and angle, with the field rotated in the a - b-plane, at temperatures between 4.2 and 30 K and in fields of up to 20 T. We interpret our results within a mean-field model and derive expressions for the basal-plane anisotropy energy. Further, we studied the anisotropy and temperature dependence of the metamagnetic transition of at 18 T and we discuss its nature.