An enumeration of different physiological microbial types detected in the groundwaters of four geothermic sites dating from the Dogger and Trias periods that make up the Paris Basin reveals a predominance of anaerobes over aerobes and thermophiles over mesophiles. The main biological activity is performed via chemolithotrophic metabolism. The anaerobic chemolithotrophic thermophiles are well adapted to an aquifer environment and are capable of a metabolic activity in the aquifer. More important, such microflora can cause biocorro‐sion.