The covalent reduction of the hyperfine splitting A of Mn2+ and other transition metal impurities in crystals is interpreted as resulting from the exchange polarization of the bonding orbitals containing 4s-admixture of the impurity. The calculated unpaired spin density responsible for the reduction of A is composed of two contributions. The first one results from the ligand → 4s transfer of the up-spin electron overcompensating the transfer of the down-spin electron. The second contribution is due to the difference between the 4s-radial wave function of up- and down-spin electrons. The latter effect is shown to be a dominating one on both the theoretical and experimental grounds. The theory is applied to discuss the relationship of A vs. covaleney for 3d transition metal ions in various hosts. It is concluded that in covalent crystals the 4s-bonding of the impurity dominates over the 3d-bonding. © 1970.