Cu/Zn superoxide dismutase (SOD1) is an abundant, largely cytosolic, enzyme that scavenges superoxide anions. The biological role of SOD1 is somewhat controversial since superoxide is thought to largely arise from the mitochondria where a second SOD (manganese SOD) already resides. Using bakers yeast as a model, we demonstrate that Cu/Zn SOD1 helps protect mitochondria from oxidative damage, as sod1? mutants show elevated protein carbonyls in this organelle. In accordance with this connection to mitochondria, a fraction of active SOD1 localizes within the intermembrane space (IMS) of mitochondria together with its copper chaperone, CCS. Neither CCS nor SOD1 contain typical N-terminal presequences for mitochondrial uptake, however the mitochondrial accumulation of SOD1 is strongly influenced by CCS. When CCS synthesis is repressed, mitochondrial SOD1 is of low abundance, and conversely, IMS SOD1 is very high when CCS is largely mitochondrial. The mitochondrial form of SOD1 is indeed protective against oxidative damage since yeast cells enriched for IMS SOD1 exhibit prolonged survival in stationary phase, an established marker of mitochondrial oxidative stress. Cu/Zn SOD1 in the mitochondria appears important for reactive oxygen physiology and may have critical implications for SOD1 mutations linked to the fatal neurodegenerative disorder, amyotrophic lateral sclerosis.