Because of their nondeterministic nature of behaviour, the supervisory control of manufacturing systems must be carried out in closed loop. This trait greatly increases the size and complexity of the discrete-event system (DES)-based supervisory-controllers of manufacturing systems. In an attempt to cope with this complexity, a hybrid supervisory controller (HSC) is proposed. It splits operations between its three elements: (i) a DES supervisory controller, which contains the nominal control strategy, (ii) a diagnostic system, which monitors the workcell, and (iii) an alternate strategy driver (ASD), that asserts control by revising the nominal strategy of the DES supervisory controller, whenever events diverge from the states of the DES supervisory controller. This modified approach to DES control provides a more efficient controller than could reasonably be attained using solely a DES supervisory controller.>