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[en] In this work, the effect of doping by tetravalent Ce"4"+on the structural and magnetoresistive properties of La_0_._7_−_xCe_xCa_0_._3MnO_3 polycrystalline manganites is presented. Samples with x=0, 0.1, 0.2 and 0.3 are prepared by solid state reaction method and characterized. For all doping levels, the X ray diffraction (XRD) analysis shows that the doping element is not completely soluble in the manganite. The cell parameters values, which decrease with Ce content, are refined in the orthorhombic system by taking into account the unreacted CeO_2 phase. The observation by scanning electron microscopy (SEM) reveals a change in the morphology of the grains and the porosity of the samples by doping. The temperature dependence of magnetization curves shows a transition from paramagnetic to ferromagnetic state in all samples. A decrease of the Curie temperature T_c with doping is revealed. The resistivity of the samples increases with Ce content and shows a clear metal to insulator transition when measured as a function of temperature. The doping also decreases the temperature T_p of this transition, in accordance with the evolution of T_c values. The calculated magnetoresistance (MR) for a magnetic field of 5T increases gradually from 39.52% for x=0 to 66.18% for x=0.3.