An optical probe for measuring the motion and rate of growth of the early flame kernel in spark ignition engines is described. The probe consists of a standard spark plug with eight optical fibers installed in a ring at the base of the threaded region of the plug. The fibers collect the light emitted from the flame as it crosses the field of view of the fibers, and transmit the light to photomultiplier tubes. The time from ignition until detection of the flame is used to compute the average flame velocity in the direction of each fiber relative to the spark location. The real-time data acquisition system permits statistical analysis of cycle-by-cycle variations in the combustion rate. Because the probe was built using a standard 14 mm spark plug, it can be used in unmodified production automotive engines.To demonstrate this device, measurements are presented comparing the flame kernel evolution between unscavenged and scavenged engine cycles, and between low-swirl and high-swirl in-cylinder flow fields. Laser schlieren visualization of the combustion process is used to validate the response of the probe. It is shown that both the displacement and growth rate of the flame kernel can be measured, and that small effects related to the orientation of the ground electrode can be sensed, Finally, two parameters characterizing the average convection velocity and expansion speed are proposed for correlating early flame development with engine pressure measurements.