High-field, multifrequency EPR data are presented for the alum CsMn(SO4)(2).12D(2)O, containing the [Mn(OD2)(6)](3+) cation. The data are interpreted using the conventional S = 2 spin Hamiltonian, and the following parameters determined for the data obtained below 30 K: D = -4.491(7) cm(-1), E = 0.248(5) cm(-1), g(x) = 1.981(5), g(y) = 1.993(5), g(z) = 1.988(5). Although the deviation of the MnO6 framework from idealized DO symmetry is small, the magnitude of E/D is significant. The E parameter is related to ligand field parameters derived from the optical absorption spectrum. The rhombic anisotropy is shown to arise as a consequence of the pi-anisotropic nature of the manganese(III)-water interaction.