A method for determining the flame contour based on the flame arrival time at the fiber optic (FO) spark plug and at the head gasket ionization probe (IP) locations has been developed. The experimental data were generated in a single-cylinder Ricardo Hydra spark-ignition engine. The head gasket IP, constructed from a double-sided copper-clad circuit board, detects the flame arrival time at eight equally spaced locations at the top of the cylinder liner. Three other IP's were also installed in the cylinder head to provide additional intermediate data on flame location and arrival time. The FO spark plug consists of a standard spark plug with eight symmetrically spaced optical fibers located in the ground casing of the plug. The cylinder pressure was recorded simultaneously with the eleven IP signals and the eight optical signals using a high-speed PC-based data acquisition system.Based on these flame arrival times, the development of the flame radius in each direction from the center of the spark plug toward each head gasket IP (Ri=f(time)) was estimated using a cubic spline interpolation. The flame contour for a given time is then drawn from a spline interpolation. The engine was run at various spark timing settings, speeds, and relative air-fuel ratios for two distinct flow configurations. This curve fitting methodology produces reasonable flame shape contours and therefore provides a useful tool for studying cycle-to-cycle variations in the combustion rate as well as the effect of different flow characteristics on flame development and end-gas region geometry and location.