A systematic study of the A-site doping in Mn(IV)-rich perovskite manganites Ca1-xHoxMnO3, over a large homogeneity range (0.1 <= x <= 0.4), has been performed. A significant increase in the lattice parameters indicated the presence of mixed valence state of Mn: Mn3+ and Mn4+. The substitution of calcium by holmium also induces strong changes in the electrical properties. We found that small Ho concentration produces an important decrease in the electrical resistivity and induces an electrical transition, the temperature corresponding to the metal-insulator transition (T-M1) shifts with the holmium content. This electrical behavior is attributed to the Mn3+ ions content and a charge order effect. (C) 2007 Elsevier B.V. All rights reserved.