Abstract: A diesel fuel tank has an opening in the top wall through which an electric immersion heater assembly is inserted into the tank to heat the contents thereof. The immersion heater assembly includes an outer tubular member arranged vertically in the tank and having an open upper end sealingly secured in the tank opening and a closed lower end near the bottom of the tank. An inner tubular member of slightly smaller diameter than the outer member and of a length coextensive with a major portion of the length of the outer member is removably inserted into the outer member through the open end thereof. A double filament DC light bulb mounted on the lower end of the inner member serves as an electric heating element located in the lower portion of the outer member. Air heated by the light bulb flows upwardly by convection into the narrow circumferential space between the inner and outer members thereby assuring that the entire length of the outer member in contact with the diesel fuel in the tank is heated.