A gene encoding 57 102 Da polypeptide homologous to groEL of Escherichia coli but accompanying no groES, has been cloned and sequenced from a thermophilic cyanobacterium, Synechococcus vulcanus. The amount of the gene transcript increased several folds by heat shock. The gene was expressed as a minor component of two types of HSP60, and designated as groEL2. Although expressed and induced well upon heat shock treatment in the E. coli, introduction of the cloned groEL2 gene of S. vulcanus into an E. coli groEL-less mutant did not result in the complementation of heat sensitivity.