Measurements have been made by the Endor technique on the trivalent chromium ion 53Cr present as a dilute substitutional impurity in a magnesium oxide crystal. The energy levels of the ion are described by the spin Hamiltonian:[Formula: see text]Measurements have been made along the cube axes of the crystals and at 45° to the cube axes in a plane that contains two cube axes. Energy levels in these directions are predicted within a probable error of 3 × 103 Hz. The values of the parameters in the spin Hamiltonian have been found to be:[Formula: see text]The operators used in this Hamiltonian are derived and discussed.