Measurements have been made by the Endor technique on one-third of the hyperfine splittings of the divalent manganese ion 55Mn2+, present as a dilute substitutional impurity in a magnesium oxide crystal; the measured transitions were those associated with the electronic quantum numbers M = + 1/2 and M = −1/2. The measured energy levels of the ion are predicted to within a probable error of 6.4 × 103 Hz by the following spin Hamiltonian:[Formula: see text]This Hamiltonian is regarded as tentative, useful until further experimental results are available.Measurements were made along the cube axes of the crystals, at 31.7° to the cube axes, and at 45° to the cube axes in a plane that contains two cube axes. The values of the parameters in the spin Hamiltonian have been found to be:[Formula: see text]