The 1991 Federal and state heavy-duty engine emission standards, and the South Coast Air Quality Management District (SCAQMD) potential requirements for fleet use of cleaner fuels, have motivated the Orange County Transportation Authority (OCTA) to evaluate clean fuels for use in transit vehicles. The objective of this project is to operate transit buses on various clean fuels to identify the best approach for OCTA's operations. Technologies tested to date are methanol with the ignition improver Avocet, compressed natural gas (CNG), liquefied petroleum gas (LPG), and diesel used in Gillig Phantom buses equipped with Cummins L10 engines. Six clean-fuel buses have been operating in service since August 1990.This paper includes an evaluation of bus operating data and exhaust emissions. Data are being collected on all aspects of bus operation, including fuel economy, fuel prices, maintenance, and road calls. Initial evaluations of exhaust emissions have been performed on a chassis dynamometer at the Los Angeles County Metropolitan Transportation Authority (MTA). Oxides of nitrogen (NOx), carbon monoxide (CO), hydrocarbons (HC), particulate (PM), and formaldehyde emissions were measured in g/mi over the Central Business District (CBD) Cycle.