The hydrocarbon, carbon monoxide, and oxide of nitrogen emission contributions of motorcycles are compared with the emission contribution of light duty vehicles, light duty trucks, and heavy duty vehicles nationwide and for six different air quality control regions (AQCRs). The AQCRs were selected as representative of those regions having trouble meeting national ambient air quality oxident and/or carbon monoxide standards. The analysis indicated that uncontrolled motorcycles alone are a relatively small percentage of the present total mobile source emissions in all AQCRs emissions. However, uncontrolled motorcycles do represent a significant percentage of the maximum allowable emissions in the AQCRs assessed. As controls of other mobile sources (namely light duty vehicles) become effective, the motorcycle if left uncontrolled would make a significant contribution to the air pollution problem.