A condition for attaining the maximum values of thermoelectric power Z*in intermetallic semiconductors of the MgAgAs structural type consists in heavy doping of these semiconductors with donor and/or acceptor impurities with concentrations at which the Fermi level is pinned at the mobility threshold for the corresponding band of continuous energies. The maximum of the thermoelectric power is thermally stable under the condition that the impurity type and the type of the impurity band that give rise to the maximum in Z* are the same.