Soot and fuel distributions have been studied in an optically accessible direct-injection diesel engine of the “heavy-duty” size class. Laser-induced incandescence (LII) was used to study the effects of changes in the engine speed on the in-cylinder soot distribution, and elastic (Mie) scattering and laser-induced fluorescence (LIF) were used to examine the fuel distribution. The investigation showed that, in this engine, soot is distributed throughout the cross section of the combusting region of the fuel jet for engine speeds ranging from 600 to 1800 rpm. No indication was found that soot occurs preferentially around the periphery of the plume. The LII images showed that the soot concentration decreases with increasing engine speed and injection pressure, and that the soot distribution extends much further upstream (toward the injector) at the lower engine speeds than at higher speeds. Fuel distribution images showed that for the conditions of this investigation, virtually all the liquid fuel has vaporized by the time it travels 35 to 45 mm from the injector, and all the fuel in the main combustion zone is vapor phase. This limited liquid penetration length indicates that it is unlikely that wall impingement of liquid fuel occurs in engines of this type at typical warm-engine operating conditions. Finally, the shape of the vapor portion of the fuel jet under noncombusting conditions is shown to closely resemble the shape of the soot distribution under fired conditions.