The emissions of two ultralow NOx heavy-duty (HD) vehicles equipped with 0.02 g/bhp-hr low NOx natural gas (NG) engines were evaluated on a chassis dynamometer. This included a waste hauler and a city transit bus, each with a 0.02 g/bhp-hr NOx L9N near zero (NZ) natural gas engine. The vehicles were tested over a variety of different cycles, including the Urban Dynamometer Driving Schedule (UDDS), port drayage cycles, transit bus cycles, and a refuse truck cycle.For both vehicles, the NOx emissions results were below the 0.02 g/bhp-hr level for most cycles, with the exception of some cold start tests. For the waste hauler, NOx emissions averaged between 0.014 and 0.002 g/bhp-hr for the hot start tests, and from 0.043 to 0.014 g/bhp-hr for the cold start tests. This represented NOx emissions reductions from 97%-100% of compared with previous ISL G 8.9 engines. For the transit bus, the NOx emissions ranged from 0.0007 g/bhp-hr to 0.0042 g/bhp-hr for the warm tests and up to 0.04 g/bhp-hr for the cold start tests. The NOx results for the warm tests are 99% lower than the existing 2010 NOx diesel standard (0.2 g/bhp-hr) and 90% lower than the optional low NOx standard (0.02 g/bhp-h). In contrast, some elevation of ammonia emissions was observed for both vehicles, due to reactions that occur over the three way catalyst. Overall, the results suggest that ultralow NOx NG engines could play an important role in reducing NOx emissions from heavy-duty vehicles towards near zero levels in urban areas.The particle mass emissions were low and typically were more than 90% lower than the 2010 certification standard (10 mg/bhp-hr) for the L9N engine for both applications. Particle number (PN) emissions for the L9N (0.02 g/bhp-h) and other previous tests of ISL G 8.9 (0.2 g/bhp-h) engines both show higher PN emissions compared to diesel vehicles equipped with diesel particle filters (DPFs). Fuel economy, greenhouse gas and nitrous oxide (N2O) emissions are also reported in this paper.