Yeast strains that lack mitochondrial function are sensitive to oxidative stress caused by reactive oxygen species (ROS). Specifically, rho0 mutants that lack mitochondrial DNA, and strains deleted for the nuclear genes COX6 and COQ3 that are required for function of the respiratory electron transport chain, were sensitive to H2O2. In addition, treatment with mitochondrial inhibitors including antimycin A, oligomycin, potassium cyanide and sodium azide increased sensitivity to H2O2. The mechanism does not appear to depend on the antioxidant status of the cell since respiratory-deficient strains were able to mount an inducible adaptive response to H2O2. We suggest that the oxidant sensitivity is due to a defect in an energy-requiring process that is needed for detoxification of ROS or for the repair of oxidatively damaged molecules.