We have analyzed the dynamics of the chaperonin (GroEL)-cochaperonin (GroES) interaction at the single-molecule level. In the presence of ATP and non-native protein, binding of GroES to the immobilized GroEL occurred at a rate that is consistent with bulk kinetics measurements. However, the release of GroES from GroEL occurred after a lag period (similar to3 s) that was not recognized in earlier bulk-phase studies. This observation suggests a new kinetic intermediate in the GroEL-GroES reaction pathway.