Natural gas has been the subject of growing interest as a low emissions alternative to conventional automotive engine fuels. The development of a control system for a very low emissions heavy-duty natural gas engine is described. The engine is intended for city bus applications, with emissions targets set well within US 1994 levels. The engine uses a stoichiometric air-fuel mixture with exhaust gas recirculation and a three-way catalyst.The control system was implemented on a prototype hardware architecture designed to facilitate algorithm development. The control system software was constructed from a number of fundamental modules.Good steady-state and transient air-fuel ratio control was particularly important for maintaining optimum catalyst efficiency and hence minimum emissions. To achieve this, the air-fuel ratio control system used solenoid gas injectors and lambda feedback. The air-fuel ratio control algorithm design involved the use of advanced dynamic modelling and system identification techniques.Other functions performed by the control system were exhaust gas recirculation (EGR) control, ignition control, knock control, boost control and idle speed control.The engine was run over the US Federal Heavy Duty Transient test and good control system performance was achieved, resulting in very low emissions. Transient torque response and stability, important driveability factors, were also good.