Much attention has focused on the aetiology of oxidative damagein cellular and organismal ageing^,,,. Especially toxic arethe reactive oxygen byproducts of respiration and other biological processes. A mev-1 (kn1 ) mutant of Caenorhabditis elegans has been found to be hypersensitive to raised oxygen concentrations,. Unlike the wild type, its lifespan decreases dramatically as oxygen concentrations are increased from 1 to 60% . Strains bearing this mutation accumulate markers of ageing (such as fluorescent materials and protein carbonyls) faster than the wild type,. We show here that mev-1 encodes a subunit of the enzyme succinate dehydrogenase cytochrome b , which is a component of complex II of the mitochondrial electron transport chain. We found that the ability of complex II to catalyse electron transport from succinate to ubiquinone is compromised in mev-1 animals. This may cause an indirect increase in superoxide levels, which in turn leads to oxygen hypersensitivity and premature ageing. Our results indicate that mev-1 governs the rate of ageing by modulating the cellular response to oxidative stress.