Yeast mutants resistant to ethidium bromide have been isolated among sensitive grande cells (ϱ+) for their ability to grow on glycerol in the presence of the dye. Mutant cells are also resistant to acriflavin and do not yield petites (ϱ-) when grown on galactose with the mutagen. Genetic analysis reveals that resistance to ethidium bromide is controlled by a cytoplasmic factor, carried by, or linked to, the ϑ determinant (mitochondrial DNA). The expression of resistance to ethidium bromide seems to be related to the presence in the cell of a product of mitochondrial protein synthesis. It is concluded that some mitochondrial DNA sequence is involved in the resistance to ethidium bromide of yeast mitochondria.