EMS is capable of inducing point mutations in mitochondrial genomes of yeast. It induces efficiently the mitochondrial suppressor mutation of the mitochondrial ochre mutation oxi 1-V25. The base changes leading to the suppression effect have not been identified. AT----GC base substitutions in mitochondrial genomes are inefficiently induced by EMS. The RAD9 and REV3 gene products participate in EMS mutagenesis in nuclear, as well as mitochondrial genomes of yeast.