In some yeast cytoplasmic mutants, ethidium bromide resistance is expressed at the mitochondrial level. In homosexual crosses it leads to an enhanced recombination between two linked mitochondrial genes controlling chloramphenicol and erythromycin resistance or sensitivity, but recombination is still reciprocal. When ethidium bromide resistant cells are irradiated with U.V., the recovery of intact markers out of irradiated mitochondrial genomes is increased by comparison with that of ethidium bromide sensitive cells.