The cyanide-insensitive superoxide dismutase of yeast has been shown to be localized in the mitochondrial matrix. This enzyme has been isolated in good yield from bakers' yeast. Its molecular weight is 96,000. It is a tetramer, being composed of four subunits of equal size. Exposure to sodium dodecyl sulfate at 100 degrees caused dissociation into dimers, while similar treatment but in the presence of 2-mercaptoethanol caused complete dissociation into monomers. This enzyme contains 1 atom of manganese per subunit and its absorption in the visible suggests Mn(III) in the resting enzyme. Ascorbate caused partial bleaching, presumably by reduction to Mn(II). The amino acid composition was determined. This enzyme has activity comparable to that of other previously reported superoxide dismutases and like the chicken mitochondrial and the bacterial enzymes, its rate of reaction with O2 falls as the pH is raised above 7.8. Crystals of high quality were easily prepared.