Cytochemical peroxidase activity in a haploid yeast ( Saccharomyces cerevisiae, strain D243) was studied in glutaraldehyde-fixed cells with a diaminobenzidine (DAB) procedure. In early stationary phase cells the predominant sites of peroxidatic activity were the mitochondrial cristae, with activity of lesser intensity on other mitochondrial membranes. The reaction required H2O2, was insensitive to aminotriazole but was blocked by cyanide and azide, by high peroxide concentration or by a decrease in medium pH from 8.5 to 6.0. The enzyme visualized was probably cytochrome c peroxidase, an enzyme known to be highly active in yeast mitochondria, although the possibility that H2O2 may somehow activate DAB oxidation via other mitochondrial heme proteins cannot be completely eliminated. The value of the DAB procedure as a means of specifically staining yeast mitochondria is discussed in relation to studies of mitochondrial biogenesis. No conclusive evidence of the presence of nonmitochondrial peroxisomal organelles (microbodies) was seen in cells grown on either glucose or galactose, but there was an indication of their presence in very low numbers in glycerol-grown yeast. Although catalase was clearly present, as evidenced by the rapid evolution of O2 from the H2O2-containing reaction media, it showed little apparent localized peroxidatic activity toward DAB. We were unable to demonstrate cytochrome oxidase activity in these cells with the usual DAB-cytochrome oxidase procedure.