Chloroplast chaperonin 20 (Cpn20) in higher plants is a functional homologue of theEscherichia coliGroES, which is a critical regulator of chaperonin-mediated protein folding. The cDNA for a Cpn20 homologue ofArabidopsis thalianawas isolated. It was 958 bp long, encoding a protein of 253 amino acids. The protein was composed of an N-terminal chloroplast transit peptide, and the predicted mature region comprised two distinct GroES domains that showed 42% amino acid identity to each other. The isolated cDNA was constitutively expressed in transgenic tobacco. Immunogold labelling showed that Cpn20 is accumulated in chloroplasts of transgenic tobacco. A Northern blot analysis revealed that mRNA for the chloroplast Cpn20 is abundant in leaves and is increased by heat treatment. To examine the oligomeric structure of Cpn20, a histidine-tagged construct lacking the transit peptide was expressed inE. coliand purified by affinity chromatography. Gel-filtration and cross-linking analyses showed that the expressed products formed a tetramer. The expressed products could substitute for GroES to assist the refolding of citrate synthase under non-permissive conditions. The analysis on the subunit stoichiometry of the GroEL–Cpn20 complex also revealed that the functional complex is composed of a GroEL tetradecamer and a Cpn20 tetramer.