It is projected that even when the entire on-road fleet of heavy-duty vehicles operating in California is compliant with 2010 emission standards of 0.20 g/bhp-hr, the National Ambient Air Quality Standards (NAAQS) requirements for ambient ozone will not be met. It is expected that further reductions in NOX emissions from the heavy-duty fleet will be required to achieve compliance with the ambient ozone requirement. To study the feasibility of further reductions, the California Air Resources Board (CARB) funded a research program to demonstrate the potential to reach 0.02 g/bhp-hr NOX emissions. This paper details the work executed to achieve this goal on the heavy-duty Federal Test Procedure (FTP) with a heavy-duty natural gas engine equipped with a three-way catalyst. A Cummins ISX-12G natural gas engine was modified and coupled with an advanced catalyst system. In addition to close-coupled and underfloor catalysts, a high energy ignition system and advanced exhaust gas recirculation (EGR)-fuel-air mixer were incorporated into the engine system. New cold start calibrations were developed, improved air-fuel ratio control was implemented, and the engine was able to demonstrate composite NOX emissions of 0.015 g/bhp-hr for the FTP, 0.002 g/bhp-hr for the RMC, and 0.008 g/bhp-hr for the World Harmonized Transient Cycle (WHTC). Additional calibration may yield further improvements for duty cycles beyond the FTP and WHTC. Results from this study demonstrate a potential pathway to achieving ultra-low NOX emissions on future heavy-duty vehicles.