Abstract: A fuel tank filler cap with superimposed compartments that are defined from one another by partition walls and filled with fibrous material, the top wall of the uppermost compartment having an air vent and the partition walls having communication ports therethrough, so located with respect to one another and the air vent that communication from the interior of the fuel tank to the air vent through the fibrous material filling the superimposed compartments is so tortuous that the escape of liquid fuel from the tank is virtually impossible, though vapors can freely leave the tank and air can readily enter it.