Much effort is currently centred on the construction of elaborate computer programs to model the release to the biosphere of radionuclides from proposed high level radioactive waste repositories in deep geological formations. Although the lack or poor quality of background data is often emphasised, it is generally considered that by examining a wide range of possible values of the important parameters involved, likely processes are assessed or their omission and possible relevence is acknowledged. However, one factor that has been almost totally ignored is the presence of micro-organisms in deep geological formations. The common assumption is that the biosphere is limited to the earth's surface, and soil to a depth of a few metres. Recent research has shown, however, that viable micro-organisms can inhabit deep groundwaters and that the biosphere can extend to depths of at least 5 km. Examples of organisms tolerant to extreme environments are given in Table 1 although ranges are likely to be conservative due to the difficulties of culturing these organisms, which tend to have exotic nutritional requirements, and sampling them in extremely hostile environments.