Fluorescence in-situ hybridization (FISH) is one of the most employed techniques for identification of microorganisms, and it can be used for quantification in complex environment. Ribosomal RNA is the target of the fluorescently labelled oligonucleotide probes used. Different studies have previously shown that hybridization signal is abruptly reduced at the beginning of the exponential growth phase. This phenomenon seems to be common to a wild range of microorganisms as yeast or bacteria. To explain this phenomenon, one can draw two main hypotheses: - a decrease in cellular ribosome content after the exponential phase onset. - a reduced probe accessibility during this bacterial growth start. Our results have shown that the amount viable cells and the DNA amount into the sample give superimposable sigmoid growth curves. For bacteria as for yeast, whereas PCR and cytometry quantification have shown a classical growth curve, the FISH signal increased suddenly at the beginning of the exponential phase. Moreover, this peak was followed by an abrupt decrease. These results confirm those previously observed. The maximum fluorescent intensity corresponds to the maximum µ value. The RT-QPCR on the total RNA has revealed that the maximum ribosome rate correspond to the maximum fluorescence intensity observed by FISH staining and to the maximum µ calculated. Our first hypothesis seems to be the most reliable. The cell division rate during the exponential phase must be most important, or at least equal, to the ribosome synthesis rate. These results clearly show that FISH is not a very efficient tool for quantification of microorganisms in complex environment.