The West Virginia University (WVU) Transportable Heavy-Duty Vehicle Emissions Testing Laboratory was employed to conduct chassis dynamometer tests in the field to measure the exhaust emissions from heavy-duty buses and trucks. This laboratory began operation in the field in January, 1992. During the period January, 1992 through June, 1993, over 150 city buses, trucks, and tractors operated by 18 different authorities in 11 states were tested by the facility. The tested vehicles were powered by 14 different types of engines fueled with natural gas (CNG or LNG), methanol, ethanol, liquified petroleum gas (LPG), #2 diesel, and low sulfur diesel (#1 diesel or Jet A). Some of the tested vehicles were equipped with exhaust after-treatment systems.In this paper, a total of 12 CNG-fueled and #2 diesel-fueled transit buses equipped with Cummins L-10 engines, were chosen for investigation. The study was based on the 1992-93 test results of the WVU Transportable Heavy Duty Vehicle Emissions Testing Laboratory. The emissions of hydrocarbons, nitrogen oxides, carbon monoxide, and particulate matter were determined. Comparisons of emissions of the vehicles, powered on Compressed Natural Gas (CNG) and on #2 diesel, showed that the carbon monoxide, nitrogen oxide, and particulate matter emissions from the CNG fueled engines were significantly lower, while the hydrocarbons were higher.