The appearance of additional states of an impurity under high doping levels is discussed. On the basis of a Fermi level calculation, a model for the conductivity of heavily boron-doped diamond is proposed. This model allows one to reproduce the temperature dependence of the conductivity and of the Hall coefficient for different impurity concentrations. The temperature dependence of the conductivity was found to be connected with the coexistence of valence-band and hopping conductivities in a wide temperature range. The hopping conductivity takes place over the additional states of the boron impurity.