There is currently a large effort in industry to make natural gas a viable alternative fuel for internal combustion engines. While the use of natural gas offers several advantages such as reduced emissions and potentially higher efficiency, it also has some inherent difficulties. Among these is the challenge of producing a consistently homogeneous air/fuel mixture while retaining the advantages which accompany modern, multi-point, fuel injection systems. The purpose of the research described here is to investigate the in-cylinder mixture formation process in a port injected natural gas fueled engine. Planar laser-induced fluorescence has been used to produce qualitative air fuel ratio maps in the engine cylinder, in selected planes, throughout the intake and compression strokes. The process consists of impinging a sheet of ultraviolet laser light on various planes parallel to, and normal to, the cylinder axis. Fluorescence results from a small amount of acetone doped into the incoming fuel. The laser pulse is of sufficiently short duration (∼10 ns) to provide essentially instantaneous images of the fuel distribution. Concentration measurements are made by collecting these planar images with an intensified CCD camera. Collected images are corrected for such factors as background noise, non-linear optics, and cylinder pressure. The result of this work is a series of qualitative two-dimensional maps of air/fuel ratio as a function of crank angle throughout the mixture formation process. The preliminary distribution maps presented here reveal not only the overall mixture inhomogeneity but also suggest means by which the mixture formation process can be enhanced. Experimentation is currently under way to provide detailed air/fuel maps at idle, and to determine the impact of injection timing on the generation of a consistently homogeneous mixture.