Five plasma jet ignitor designs were tested on a Waukesha ASTM-CFR engine fueled with natural gas. The pollutant emissions, fuel and air flow rates and dynamic cylinder pressure were measured for the full range of Air/Fuel ratios. From these measurements the indicated power and specific fuel consumption were calculated. The energy for the ignitors was provided by a variable high energy ignition system, and each ignitor was supplied with 0.00, 0.08, 0.32, 0.72 and 1.28 Joules of energy in addition to that provided by the standard ignition coil. To differentiate between the benefits gained by use of the plasma jet ignitors and those due to the higher ignition energies, an ordinary spark plug was also tested with added ignition energies. The goal of the experiment was to find an ignitor that could be used to extend the lean operating limit of a natural gas fueled engine, so that the emission of NO_x, and other pollutants could be reduced. The following table shows the optimum pollutant emission reductions achieved by the use of the most effective plasma jet ignitor and the high energy spark plug compared with the emissions from the engine when operated with the standard equipment spark plug near stoichiometric. The plasma jet ignitor for which the results are displayed in this table consisted of an 83 mm³ cavity and a 118 mm³ ejector, both of which were insulated with ceramic cylinders. / Master of Science