NO formation during direct-injection (DI) diesel combustion has been investigated using planar laser-induced fluorescence (PLIF) imaging. Measurements were made at a typical medium-speed operating condition in a heavy-duty size-class engine modified for optical access. By combining a unique laser system with a particular spectroscopic scheme, single-shot NO images were obtained at realistic operating conditions with negligible O2 interference. Temporal sequences of NO PLIF images are presented along with corresponding images of combined elastic scattering and natural luminosity. These images show the location and timing of the NO formation relative to the other components of the reacting fuel jet. In addition, total NO formation was examined by integrating the NO PLIF signal over a large fraction of the combustion-chamber volume. These total NO measurements are corrected for the effects of pressure, temperature, and mixing so that a quantitative measure of the NO-formation time history is obtained.The NO PLIF data strongly support and extend the conceptual model of DI diesel combustion recently presented by one of the authors. The images show that NO is not produced by the initial premixed combustion (which is fuel-rich), but begins around the jet periphery just after the diffusion flame forms. NO formation then increases progressively throughout the remainder of the combustion event, with the NO being confined to the jet periphery until the jet structure begins to disappear toward the end of the apparent heat release. The data also show that NO formation continues in the hot post-combustion gases after the end of the apparent heat release, for the operating condition studied.