Measurements of the thermal conductivity of MgO doped with manganese in the temperature range 0.4 to 300K indicate the existence of resonant scattering by Mn³⁺ ions at about 160 and 630 GHz. This cannot be accounted for by simple crystal-field theory but is consistent with the dynamic Jahn-Teller theory of Fletcher and Stevens (1969) with the parameters delta (tunnelling frequency)=480 GHz and D (spin-orbit splitting)=120 GHz. These frequencies and parameters are the same within experimental error as those determined for the isoelectronic Cr²⁺ in MgO using the same technique by Challis et al. (1978).