The GroES heptamer forms a dome, approximately 75 Angstrom in diameter and 30 Angstrom high, with an 8 Angstrom orifice in the centre of its roof. The ’mobile loop’ segment, previously identified as a GroEL binding determinant, is disordered in the crystal structure in six subunits; the single well-ordered copy extends from the bottom outer rim of the GroES dome, suggesting that the cavity within the dome is continuous with the polypeptide binding chamber of GroEL in the chaperonin complex.