Chaperonin (Cpn) is one of the molecular chaperones. Cpn10 is a co-factor of Cpn60, which regulates Cpn60-mediated protein folding. It is known that Cpn10 is located in mitochondria and chloroplasts in plant cells. The Escherichia coli homologue of Cpn10 is called GroES. A cDNA for the Cpn10 homologue was isolated from Arabidopsis thaliana by functional complementation of the E. coli groES mutant. The cDNA was 647 bp long and encoded a polypeptide of 98 amino acids. The deduced amino acid sequence showed approximately 50% identity to mammalian mitochondrial Cpn10s and 30% identity to GroES. A Northern blot analysis revealed that the mRNA for the Cpn10 homologue was expressed uniformly in various organs and was markedly induced by heat-shock treatment. The Cpn10 homologue was constitutively expressed in transgenic tobaccos. Immunogold and immunoblot analyses following the subcellular fractionation of leaves from transgenic tobaccos revealed that the Cpn 10 homologue was localized in mitochondria and accumulated at a high level in transgenic tobaccos.