An 8.84-meter (29-foot) transit bus, powered by a phosphoric acid fuel cell (PAFC)/battery hybrid power plant, is being developed and tested under a program sponsored by the U.S. Department of Energy, the U.S. Department of Transportation, and the South Coast Air Quality Management District. When compared to diesel and other alternative fuel technologies, fuel cells offer the potential for both reduced emissions and increased fuel economy. The fuel cell/battery hybrid-powered bus is essentially a “zero emission” vehicle, but does not have the range and payload limitations of a battery-only bus.The paper describes how this developmental fuel cell technology has been integrated into an 8.84-meter (29-foot) transit bus, intended for use in a revenue service demonstration. The paper also discusses the hardware system integration process and how specific design problems have been solved, including fuel cell subsystem design; battery selection and sizing; on-board energy management; and heating, ventilation, and air conditioning (HVAC) subsystem design. The paper concludes by discussing how the system integration process can be applied to other heavy-duty, hybrid vehicles.