Abstract: Apparatus is disclosed for discriminating between desired and undesired fluids such as kerosene. The apparatus is especially adapted for improving the safety of kerosene heaters by discriminating between kerosene suitable for heater use and more volatile materials such as gasoline. Structure is disclosed to terminate flow of fluid to a kerosene heater in response to the sensed presence of a fluid other than kerosene. According to a preferred form of the apparatus, additional structure is provided to prevent fueling a kerosene heater by structure other than that operatively associated with fluid discriminating structure. According to the more specific aspects of the fluid discriminator, a float including magnetic or light-responsive actuators is calibrated to permit or cease the supply responsive to the position of the float in the fluid. A fluid less dense than kerosene, such as gasoline, will cause the float to sink within the fluid and thereby terminate the flow of fluid to the kerosene heater.