Recent investigations have discovered surprisingly high numbers of microorganisms in shallow water-table aquifers. Evidence is accumulating that these microorganisms may, under certain circumstances, transform many of the organic pollutants that enter the subsurface environment. These transformations can lead to total destruction of the pollutant or to the production of new organic pollutants. The research effort in this area is expanding rapidly. As our knowledge grows, microbiology should become a useful complement to the earth sciences in our search for a better understanding of the behavior of organic contaminants in the subsurface environment.