HEATSHOCK protein 60 (hsp60) in the matrix of mitochondria is essential for the folding and assembly of newly imported proteins1. Hsp60 belongs to a class of structurally related chaperonins found in organelles of endosymbiotic origin and in the bacterial cytosol2-9. Hsp60 monomers form a complex arranged as two stacked 7-mer rings6. This 14-mer complex binds unfolded proteins at its surface, then seems to catalyse their folding in an ATP-dependent process10. The question arises as to how such an assembly machinery is itself folded and assembled. Hsp60 subunits are encoded by a nuclear gene and translated in the cytosol as precursors7 which are translocated into mitochondria and proteolytically processed. In both intact cells and isolated mitochondria of the hsp60-defective yeast mutant mif4, self-assembly of newly imported wild-type subunits is not observed. Functional pre-existing hsp60 complex is required in order to form new, assembled, 14-mer. Subunits imported in vitro are assembled with a surprisingly fast half-time of 5-10 min, indicative of a catalysed reaction. These findings are further evidence that self-assembly may not be the principal mechanism by which proteins attain their functional conformation in the intact cell.