The occurrence of viable microorganisms in 49 samples taken from 8 large tanks was examined quantitatively and qualitatively. Different contents of bacteria and fungi have been found in 16 of 25 samples near the tank-bottoms, mainly in that from kerosene- and diesel oil-tanks. The maximum count is up to 2.7×107/ml for bacteria, 8.0 × 104/ml for fungi. Liquid paraffin oxidizer, sulfate-reducer and iron bacteria are found only in several samples.On the basis of microbiological identification some bacterial is...