Natural gas (NG) is attractive for heavy-duty (HD) engines for reasons of cost stability, carbon emissions, and fuel security. NG cannot be reliably compression-ignited, but conventional gasoline-engine ignition systems are not optimized for NG and are challenged to ignite mixtures that are lean or diluted with exhaust-gas recirculation (EGR). NG ignition is particularly challenging in large-bore engines, where completing combustion in the available time is more difficult. Using two high-speed infrared (IR) cameras imaging through borescopes mounted in one cylinder of an HD NG engine, the effect of ignition systems on the early flame-kernel development and cycle-to-cycle variability (CCV) was investigated for mixture conditions ranging from stoichiometric and undiluted to lean and diluted. Imaging in the near and short-wavelength IR yielded strong signals from the water emission line at 1.3 μm, which locates the flame front and burned-gas regions and obviates intensification (which can reduce spatial resolution). The 9.7-liter inline-six engine was modified to enable exhaust-gas recirculation and to provide optical access. Two ignition technologies were studied: a high-energy conventional system using coils that delivered 140 mJ of energy to each spark and a Bosch Controlled Electronic Ignition (CEI) system. CEI uses electronics to extend the ignition event, yielding sparks up to 3 ms in duration with up to 300 mJ of energy. Air/fuel ratios as high as 1.75 (without EGR) and EGR fractions as high as 26% (stoichiometric) were reliably ignited. Both systems showed increased CCV for more lean/dilute conditions, but, for a given mixture, CEI exhibited less CCV than the conventional system. Additionally, CEI was able to ignite leaner and more dilute mixtures for a given CCV level. The images and quantitative image-based measurements correlated well with measurements made using in-cylinder pressure transducers.