Results of electronic structure calculations of C6o and M3C6o (M = K, Rb, Cs) fcc crystals carried out by the full-potentiallinear muffin-tin orbital (LMTO) method are reported. Undoped C6o is found to be semiconductir~g wtt:z a direct gap between the fully occupied valence band and empty conduction band of 0.75 eV at the X-point. The Fermi level in M3C6o falls on the haft-occupied peak formed mainly by the 2p states of the closest carbon atoms with neighboring C6o molecules. E~ates of electron-phonon coupling using the crude rigid iron approximation give A values of 0.51, 0.61 and ~.72 for IC Rb, a:d Cs doping, respectively. Using an average phonon frequency of 1100 K the McMillan formula gives T~ values of 16 K for K3C6o, 30 K for Rb3C6o and 47 K for Cs3C6o. The calculated Tc values for K and Rb doping are in very good agreement with experiment and the predicted T~ value of Cs3C6o L ~lose to the value obtained by linear extrapolations based on tb-!z~ce constant. These are the first calculated results for 3. and Tc and they strongly support the idea that BCS superconductivity is caused by the electron-phonon interaction and that Tc changes are caused by negative chemical pressures effects resulting in the increase of lattice constaats in the K, Rb, and Cs series.