Summary The electric charge stoichiometry of calcium ion transport into respiring mitochondria has been measured, using H + - and Ca 2+ -sensitive electrodes to determine the number of H + ions ejected in electroneutral exchange for Ca. In three types of system the ratio H + ejected/Ca 2+ taken up was very close to 2.0 when phosphate movements are prevented. Thus Ca is transported into respiring mitochondria in an electrogenic process in which each Ca carries 2 positive charges. The conclusion of Moyle and Mitchell (FEBS Lett. 73 , 131–136, 1977) that Ca carries only one (+) charge is shown to be based on inadequate data and assumptions.