This article reports on an investigation of the damage to fuels caused by microorganisms in various climatic zones. Fuel samples were taken from aircraft, ships, and tractors. The results indicate that the most susceptible to biological damage are diesel fuels in shipboard equipment operating in regions with a warm, moist (tropical) climate. The biological damage of fuels can lead to the plugging of filters and other components, interference with the operation of the sensing elements of fuel gages and fuel control equipment, damage to the internal coatings and the corrosion of fuel tanks, filter housings and parts, fuel meters, and other components. It is determined that in regions with a warm, moist (tropical) climate, rapid growth of microbes may occur, leading to contamination of the fuels with biomass, considerable increases in the fuel acidity, and acidification of the water accumulated under the fuel in aircraft fuel tanks and storage tanks. No biological damage to fuels is found to occur in regions with a temperate climate.