The electrical conductivity of (Mg,Fe)O magnesiow¿ustite containing 9 and 27.5 mol % FeO has been measured at simultaneously high pressures (30--32 GPa) and temperatures using a diamond anvil cell heated with a continuous wave Nd:YAG laser and an external resistance heater. The conductivity depends strongly on the FeO concentration at both ambient and high pressures. At the pressures and temperature of about 30 GPa and 2000 K, conditions expected in the lower mantle, the magnesiow¿stite containing 27.5% FeO is 3 orders of magnitude more conductive than that containing 9% FeO. The activation energy of magnesiow¿stite decreases with increasing iron concentration from 0.38 (¿0.09) eV at 9% FeO to 0.29 (¿0.05) eV at 27.5% FeO. ¿ American Geophysical Union 1990