  Electron Spin Resonance and optical reflectivity measurements demonstrate a
metal-insulator transition in Na_2CsC_60 as the system passes from the low
temperature simple cubic to the high temperature {\it fcc} structure above 300
K. The non-conducting electronic state is especially unexpected in view of the
metallic character of other, apparently isostructural fullerides, like K_3C_60.
The occurence of this phase in Na_2CsC_60 suggests that alkali specific effects
can not be neglected in the description of the electronic properties of alkali
doped fullerides. We discuss the origin of the insulating state and the
relevance of our results for the anomaly observed in the magnitude of the
superconducting transition temperature of Na_2AC_60 fullerides.
