The mixture formation process in a stratified-charge spark-ignition engine under moderate load conditions has been determined from fuel concentration measurements obtained from planar laser-induced fluorescence (PLIF) of a fluorescence fuel marker, 3-pentanone. A one-cylinder 4-valve engine, specifically designed to provided optical access through the walls of the combustion chamber and through a piston window, was used. In order to gain insight into the processes influencing the fuel motion and mixing, average fuel concentrations were recorded in four different planes between 0.7 mm to 15 mm below the spark plug for various crank-angle positions during the inlet and compression stroke and for two different injection timings. These measurements give a mean 3-dimensional picture of the time history of the fuel distribution. The results show the presence of an axial fuel stratification in the cylinder up to the time of ignition, the degree of this stratification was found to be a function of injection timing. Fuel impinging on the cylinder wall, opposite to the inlet valves, was found to have a important influence on the mixture formation process in this type of engine geometry.