Four‐electrode conductivity measurements over the temperature range from 500 to 1000°K have been made on clear BaO single crystals and on crystals colored blue by excess barium densities between 0.3 and 6×1017per cc. Barium was introduced into the crystals by heating in a barium atmosphere and the excess barium densities were computed by the Smakula formula from the 2.0‐ev optical absorption band following the method of Sproull, Bever, and Libowitz. The conductivities of the clear crystals attain values of 10−4mho/cm at 1000°K whereas the blue crystals show a conductivity maximum of about 1 mho/cm at about 900°K. The assumption of an electron mobility of 5 cm2/volt sec leads to an agreement of the saturation electron density and the excess barium density to within a factor of three.