The complex E.P.R. spectrum of ferroelec. Mg3B7O13Cl natural crystals at 25 Deg contains a strong hyperfine sextet ascribed to Mn2+ substituted for Mg2+ ions. By using S = 5/2, I = 5/2, and g = 2; spin Hamiltonian consts. in cm.-1 are: |D| = 0.30 +- 0.02, |E| = 0.051 +- 0.005, |Ax| = | = | = 75.5 +- 2. * 10-4, and |Ax| = 82 +- 2. * 10-4. The very large D value is attributed to crystal field anisotropy from 2 types of ligands in the coordination octahedron.