Abstract: A solid state fuel heater unit for use in heating the fuel flow in a fuel filter is disclosed which unit includes a heat conducting ring having a power transistor mounted thereon and driving a power resistor mounted therewithin. Circuitry for controlling the operation of the power transistor, including a thermistor, is mounted within the ring by means of component sealant or potting material which isolates the thermistor from direct transfer of heat thereto from the power transistor and power resistor. The heater unit is adapted to be immersed in the flow of fuel in a fuel filter with the axis of the ring substantially parallel to the fuel flow through the filter.