Double B mixed perovskite-type series in the La_0.6Ca_0.4Mn_1-xCo_xO_3-δ system with x = 0-1.0 was synthesized in air by solid state reactions at 1100 °C from La₂O₃, MnO₂, CoO simple oxides and CaCO₃. Replacement of Mn cations by Co is characterized by U-shaped dependence of the conductivity versus Co content with a small maximum at x = 0.3. Lower conductivities of the Co-Mn containing compositions compared to La_0.6Ca_0.4MnO_3-δ and La_0.6Ca_0.4CoO_3-δ are explained by stronger polarization of the double B ⋯ -Mn^(α+γ)+-O^α--Co^(α-γ)+-O^α-- ⋯ fragments of the ⋯ -O^α--Mn^α+-O^α--Mn^α+-O^α--Mn^(α+γ)+-O^α--Co^(α-γ)+-O^α-- ⋯ chains in comparison with the single B ⋯ -O^α--Mn^α+-O^α--Mn^α+-O^α-- ⋯ and ⋯ -O^α--Co^α+-O^α--Co^α+-O^α-- ⋯ fragments, because of different electronegativity values of Co and Mn. The double exchange theory gives strong explanation for this phenomenon. Type and level of the electrical conductivity of the La_0.6Ca_0.4Mn_1-xCo_xO_3-δ series are functions of the [Mn⁴⁺]/[Mn³⁺] and [Co³⁺]/[Co²⁺] ratios. Mn⁴⁺ and Co²⁺ cations are possible point defects which determinate the observed p- or n-type conductivity of the compositions, respectively.