In a review with many refs. the influence of microorganisms on the corrosion of ferrous metals used in petroleum industries is described. The microorganisms, which could be bacteria, fungi or lichen, used Fe and Fe2+ or H2S as electron donors and needed a carbon source for growth, and the release of H+ enhanced corrosion. By the selection of the ferrous material, and by biol., chem. and phys. countermeasures the microbial corrosion could be protected.