Parallel-mode X-band EPR spectra of the manganese(II) hexaaqua ion as substitutional impurity in Cs[Mg(H₂O)₆]AsO₄ are presented and interpreted. In this lattice the aqua ion, considering also the disposition of the hydrogen atoms, occupies a crystallographic site with tetrahedral symmetry. This rare situation enables that the line positions as well as the intensities in the parallel-mode EPR spectra can be modelled with a simple three-parameter cubic-symmetry spin Hamiltonian, including the isotropic Zeeman interaction (g) pertinent to the S = 5/2 electronic spin, the isotropic hyperfine interaction (A), between the electronic spin and the I = 5/2 nuclear spin, and the cubic splitting (a) of the S = 5/2 manifold. A simple formalism, based on perturbation theory to grossly account for the observed intensities, is presented. We observe all parallel-mode-allowed hyperfine transitions associated with all fine-structure transitions.