Heavy carbon doping of MgB 2 is studied by first principles electronic structure studies of two types, an ordered supercell (Mg(B 1รx C x ) 2 , x = 0.0833) and also the coherent potential approximation method that incorporates effects of B-C disorder. For the ordered model, the twofold degenerate r-bands that are the basis of the high temperature superconductivity are split by 60 meV (i.e. 7 meV/% C) and the r Fermi cylinders contain 0.070 holes/cell, compared to 0.11 for MgB 2 . A virtual crystal treatment tends to overestimate the rate at which r holes are filled by substitutional carbon. The coherent potential approximation (CPA) calculations give the same rate of band filling as the supercell method. The occupied local density of states of C is almost identical to that of B in the upper 2 eV of the valence bands, but in the range ร8 eV to ร2 eV, C has a considerably larger density of states. The calculations indicate that the r Fermi surface cylinders pinch off at the zone center only above the maximum C concentration x % 0.10. These results indicate that Mg(B 1รx C x ) 2 as well as Mg 1รx Al x B 2 is a good system in which to study the evolution of the unusual electron-phonon coupling character and strength as the crucial r hole states are filled.