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The effect of Co substitution on the crystal structure and electrical transport properties of La0.85Ag0.15Mn1−yCoyO3 compounds (0≥y≤0.50) has been studied. Structural transition from rhombohedral to orthorhombic symmetry has been observed with Co doping. The lattice parameters are found to increase with doping up to y=0.20, then it decreases. It is explained on the basis of transition from high spin state to low spin state of Co ions with increase in doping beyond y=0.20. Ferromagnetic (FM) metallic behavior with colossal magneto-resistivity has been observed up to y=0.10. However, for y≥0.15 compounds, the temperature dependence of resistivity ρ(T) follows semiconducting behavior. The electrical resistivity in the metallic region could be explained based on electron–electron and electron–magnon scattering mechanisms. The data in the semiconducting region could be explained based on the variable range hopping model for y=0.2 and adiabatic small polaron hopping model for y≥0.3.