Abstract: Diesel fuel is heated prior to filtration by passing it in thermal contact with a laminar heater immersed in the fuel. The heater has a resistance at room temperature of 0.30 to 0.75 ohm and comprises a PTC conductive polymer resistive heating element sandwiched between equispaced imperforate metal foil electrodes, particularly electrodeposited foil electrodes, coextensive with resistive heating element. The conductive polymer has a resistivity at room temperature of 20 to 2000 ohm - cm. The resistive element has a cross-sectional area in a plane parallel to the electrodes of 3 to 10 square inches and the electrodes have a microrough surface in direct physical contact with the conductive polymer.