The high-pressure direct injection (HPDI) of natural gas (NG) permits diesel engines to retain their high fuel economy while reducing regulated emissions. In the work presented in this paper, directly injected natural gas is ignited by pilot diesel fuel, and both fuels are injected through a single injector. The injector concept is discussed, along with the description of the instrumented Detroit Diesel two-stroke 6V-92TA DDEC II engine used for the experiments. Measurements of the performance and emissions with the HPDI of NG confirm the retention of the high efficiency of the diesel engine and demonstrate reductions in nitrogen oxide (NOx) emissions near 50% at high load using the same injection timing as for diesel fueling. Methane (CH4) and non-methane hydrocarbon (nmHC) emissions were found to be as low as those measured for diesel fueling at high loads, but were higher at low load operation. The gas injection pressure was found to affect the low-load emissions.