The hsp60 protein isolated from the protozoan Tetrahymena thermophila is induced in response to heat stress and is a member of an immunologically conserved family represented in Escherichia coli and in mitochondria of plants and animals^1,2. We report here the cloning and characterization of a nuclear gene, HSP60, which codes for the hsp60 homologue from the yeast Saccharomyces cerevisiae. Nucleotide sequence analysis revealed that yeast hsp60 is related to the groEL protein of E. coli^3-6 and the RUBISCO-binding protein (RBP) of chloroplasts⁷. HSP60 was found to be the genetic locus of the conditional-lethal mutation described by Cheng et al.⁸, which at non-permissive temperature is defective in the assembly of several different multisubunit complexes in mitochondria. These data are consistent with the hypothesis that the groEL-related proteins serve an evolutionarily conserved function as accessory factors facilitating the folding and/or association of individual subunits of multimeric protein complexes^7,9-14.