DC-8 aircraft fuel tank sump drainings and soil, air, and fuel-water samples from fuel distribution facilities were analyzed for microbial contamination. The data were evaluated to determine the fuel tank contamination, its sources, and its effect upon the aircraft so that corrosion prevention methods could be developed. Sampling plans and microbial examination techniques are presented. Principal contaminants were Hormodendrum resinae, Candida sp., and yeastlike cells. They appeared together in many aircraft. Most of the microorganisms found in aircraft samples were also found in samples from fuel distribution facilities, but only in very low frequency in aircraft dispensed fuel. The fuel facilities could serve as an important microbial source when the final barrier is breached or not effective.