Cytochemical and ultrastructural analysis of wild-type cells of Saccharomyces cerevisiae, grown aerobically in a glucose-limited chemostat, shows that cytochrome c peroxidase is localized between the membranes of the cristae, that is, in the intracristal space. This enzyme is thus positioned appropriately within the organelle to act as an alternate terminal oxidase for the respiratory chain. The proximity of the peroxidase to major sites of generation of its two substrates may account for the small leakage of hydrogen peroxide from yeast mitochondria, as compared with the larger outflow from mammalian mitochondria. In the cytoplasmic petite mutant, gross distortion of promitochondrial membrane arrangement is evident. Nevertheless, cytochrome c perioxidase activity is present in the same amounts as is found in wild-type cells, and is localized predominantly within annuli of membrane which constitute the promitochondria in these cells. No unequivocal evidence was obtained for the localization of catalase in microbodies or other organelles in either wild-type or petite cells.