Abstract: A fuel control system for an internal combustion engine fuelled by a gaseous fuel and a liquid fuel, comprises liquid fuel flow rate control means, and gaseous fuel flow rate control means. The liquid fuel and gaseous fuel flow rate control means are adapted to control the relative proportions of each fuel supplied to the engine, and permit the relative proportion of liquid fuel to be varied in the range having an upper limit of substantially 100%. The gaseous fuel flow rate control means includes gaseous fuel flow rate determination means which is responsive to engine variables, such as engine speed and throttle position to determine from said variables a desired gaseous fuel flow rate. The gaseous fuel flow rate determination means is adapted to generate a signal indicative of said desired gaseous fuel flow rate. This signal can be used to actuate a valve controlling the gaseous fuel flow rate.