Mitochondrial DNA (mtDNA) was isolated from two hybrids of maize (Zea mays L.), NC232 ✕ T204N (normal cytoplasm) and NC232 ✕ T204 cms T (Texas cytoplasm). No difference was found in the buoyant densities of mtDNA isolated from the two hybrids. The mtDNA had a buoyant density of 1.706 g/cm3 in CsCl and a molar % guanine plus cytosine of 46.9. Therefore, the cytoplasmic dissimilarities with respect to sterility and disease response between the normal and the Texas cytoplasm cannot be accounted for by detectable alterations in the base composition of mtDNA.