Monte Carlo computational experiments were used to study the diffusion anisotropy of fast and slow neutrons in singly- and doubly-periodic, two-region reactor lattices. In two-region systems, it is shown that the separation of the anisotropy factor into a part representing mean free path discontinuity and a part representing absorption probability discontinuity is physically meaningful. In the singly-periodic lattice, the anisotropy factor for fast neutrons was greater than unity, while that for slow neutrons was less than unity. It is possible, however, for the slow neutron anisotropy factor to exceed unity in doubly-periodic lattices. The anisotropy extreme, in the singly-periodic lattice, occurred when the albedo of the moderator slab was equal to that of the fuel slab.