Ample evidence(1, 2, 3) exists to indicate that, in the presence of contaminated water and microbiological growths, the present widely used nitrile rubber compositions for lining integral fuel tanks of aircraft do not always offer satisfactory protection against corrosion. Failure of the lining sometimes occurs, which leads subsequently to pitting and inter-crystalline corrosion of the underlying aluminium alloy, resulting in structural weakening of the affected members, and in some instances actual perforation of the aircraft skin. In the majority of observed instances, the corrosion occurs beneath a section of the lining which has become deteriorated. This deterioration occurs in areas where water lies in contact with the nitrile rubber lining, especially where micro-biological slimes have developed at the interface of water, fuel and tank lining. Our evidence in kerosine systems indicates that these slimes are essentially fungal in nature, the predominant fungus being Cladosporium resinae (Lindau) de Vries, and that the associated water may range from low levels of electrolyte contamination to about 3500 ppm of salinity. We are not unmindful of the fact that numerous bacterial species have been found in such slimes, and may well contribute to the mechanism of deterioration of the lining, and the subsequent corrosion of the metal.