The effects of ambient gas thermodynamic state and fuel composition on the autoignition of natural gas under direct-injection diesel conditions were studied experimentally in a constant-volume combustion vessel and computationally using a detailed chemical kinetic model. Natural gas compositions representative of variations observed across the U.S. were considered. These results extend previous observations to more realistic natural gas compositions and a wider range of thermodynamic states that include the top-dead-center conditions in the natural gas version of the 6V-92 engine being developed by Detroit Diesel Corporation.At temperatures less than 1200 K, the experiments demonstrated that the ignition delay of natural gas under diesel conditions has a dependence on temperature that is Arrhenius in character and a dependence on pressure that is close to first order. The Arrhenius temperature dependence agrees with observations previously reported for natural gas and well-established trends for conventional diesel fuels. Natural gas composition did not change the nature of the above dependencies but did affect the magnitude of the ignition delay. The measured ignition delays were longest for pure methane and became progressively shorter as ethane and propane concentrations increased. At higher ambient temperatures (> 1300 K), the experimental ignition delays approached a limiting value that was consistent with physical delays associated with the injection system. The trends with regard to composition and pressure discussed above were in agreement with trends predicted with a detailed chemical kinetic model.