Results from work focusing on the development of an ultra low emissions, high efficiency, natural gas-fueled heavy- duty engine are discussed in this paper. The engine under development was based on a John Deere 8.1L engine; this engine was significantly modified from its production configuration during the course of an engine optimization program funded by the National Renewable Energy Laboratory.Previous steady-state testing indicated that the modified engine would provide simultaneous reductions in nonmethane hydrocarbon emissions and fuel consumption while maintaining equivalent or lower NOx levels. Federal Test Procedure transient tests confirmed these expectations. Very low NOx emissions, averaging 1.0 g/bhp-hr over hot-start cycles, were attained; at these conditions, reductions in engine-out nonmethane hydro-carbons emissions (NMHC) were approximately 30 percent, and fuel consumption over the cycle was also reduced relative to the baseline. The lowest NOx results obtained were below 0.77 g/bhp-hr. Composite cold-start/hot-start test results were also very good, with NOx at approximately 1.0 g/bhp-hr and engine-out NMHC emissions less than 0.5 g/bhp-hr.Overall, the NOx versus efficiency tradeoff for the engine was improved significantly. Also, in comparison to the California Air Resource Board ultra low-emissions vehicle (ULEV) standards, the composite test results were roughly 60 percent of the ULEV standards for NOx + NMHC, and the results for the lowest NOx cycle were below one half of the NOx + NMHC standard. Carbon monoxide and particulate matter were also well below the ULEV limits, again with no oxidation catalyst being used.