The anisotropy in the static magnetic susceptibility of bis-tetramethyltetraselenafulvalene hexafluoroarsenate [(TMTSF)2AsF6] has been investigated above and below the metal-to-insulator transition for a range of fields between 0.5 and 30 kG. The results are consistent with the expectations of a simple antiferromagnet with easy, intermediate, and hard axes close to the crystallographic b*, a, and c* directions. Below the transition temperature Tc=12 K, a spin-flop transition is seen. The temperature dependence of the spin-flop critical field Hc=4.5 kG is insignificant. Above the transition temperature, evidence for magnetic crossover is observed. The experimental results show (TMTSF)2X, X- = AsF6- and PF6- to be members of a family with quite similar physical properties. The data are discussed on the basis of Overhauser's treatment of itinerant antiferromagnetism.