Cell separation studies in conjunction with immunocytochemical studies indicate that mesophyll cells and bundle-sheath cells, the dimorphic photosynthetic cell types in the leaves of the C(4) plant Zea mays, differ in their catalase composition. In particular, catalase-2, the product of the Cat2 gene, is found primarily in the bundle-sheath cells, whereas catalase-3, the product of the Cat3 gene, is found primarily in the mesophyll cells. Electron microscopic observations reveal that bundle-sheath cells of A16, a mutant line lacking expression of the Cat2 gene in all tissues examined, contain numerous peroxisomes, but they are acatalasemic as determined by staining with 3,3'-diaminobenzidine. The significance of this mutant in physiological studies is discussed.