Organelles with the ultrastructure and cytochemical characteristics of peroxisomes (microbodies) have been identified in cells of the zona fasciculata and zona reticularis of the rat adrenal cortex. These peroxisomes appear as small, elliptical to spherical or branched structures, enclosed by a single membrane and composed of a moderately electron-dense matrix. They do not possess a nucleoid or core of the type found in peroxisomes of liver and kidney. These organelles show a strongly positive staining reaction with the diaminobenzidine technique for peroxidatic activity of catalase. This staining is inhibited by aminotriazole. In cytochemical preparations revealing acid phosphatase activity, lysosomes are strongly stained and peroxisomes are free of reaction product.