We have observed acoustically a spin-resonance line that we believe is due to the ground state of Al2O3: Fe2+. This spin system has not previously been detected by resonance techniques. The transition has a gII of 3.40±0.04, and its variation with magnetic field direction suggests that a singlet level is also present, at an energy, when the field is parallel to the c axis, of not less than 6 cm-1 above the ground doublet. From the intensity of the line, it was estimated that the magneto-elastic coefficient [14(G11-G12)2+G162]12 has a value of not less than 15 cm-1 (unit strain) -1. These results are consistent with recent calculations by Stevens and Walsh. © 1969 The American Physical Society.