Aims: The identification of the culturable and nonculturable bacterial population in ground water of a municipal water supply in Mainz (Germany) during the year 2002. Methods and Results: Total counts varied between 3AE5 · 10 3 and 2AE2 · 10 4 cells ml )1 , viable counts were approximately between 8AE1 · 10 2 and 3AE3 · 10 3 cells ml )1 . After cultivation on different nutrient media (R2A, DEV, PCA, Endo, Standard) <1% appeared to be culturable on the media used. After denaturating gradient gel electrophoreses, up to 24 different bacterial species were detected in the ground water. With the aid of 16S rDNA isolation, amplification and sequencing, the isolated organisms and clones could be identified. Conclusions:The isolated and cultured organisms mainly belonged to the Proteobacteria (a, b and c), Flavobacteria or Actinobacteria. However, most of the noncultured micro-organisms were b-Proteobacteria. Significance and Impact of the Study: This is the first study in which the identification of all culturable and nonculturable bacteria in a ground water has been attempted.