Natural gas has been used extensively as art engine fuel in gas pipeline transmission applications and, more recently, as a fuel for transportation applications including both light-duty and heavy-duty vehicles. The objective of this work was to investigate the performance and emission characteristics of natural gas in an original equipment manufacturer (OEM), light-duty, spark-ignited engine being operated in the lean fueling regime and compare the operation with gasoline fueling cases. Data were acquired for several operating conditions of speed, throttle position air-fuel equivalence ratio, and spark timing for both fuels. Results showed that for stoichiometric fueling with a naturally aspirated engine, a power loss of 10 to 15 percent can be expected for natural gas over gasoline fueling. For lean operation, however, power increases can be expected for equivalence ratios below about phi = 0.80 with natural gas fueling as compared to gasoline. Higher brake thermal efficiencies can also be expected with natural gas fueling with maximum brake torque (MET) timings over the range of equivalence ratios investigated in this work. Coefficient of variation (COV) data based on the indicated mean effective pressure (IMEP) demonstrated that the engine is much less sensitive to equivalence ratio leaning for natural gas fueling as compared to gasoline cases. The lean limit for a COV of 10 percent was about phi = 0.72 for gasoline and phi = 0.63 for natural gas. Lean fueling resulted in significantly reduced NOx levels where a lower plateau for NOx concentrations was reached at phi near or below 0.70, which corresponded to about 220 ppm. For natural gas fueling, this corresponded to about 1.21 gm/(kW-h). Finally, with MBT timings, relatively short heat release durations were obtained for lean fueling with natural gas compared to gasoline.