The microscopical organisms which float freely in the water of Lake Léman belong mainly to the Cyanobacteria, the Algae, the Ciliata, the Rotifera and the Crustacea. More than 850 species have been recorded, but a smaller number of them is regularly observed. Several measurements e.g. chlorophyll content, biovolume of samples, direct counts, provide informations on the populations' successions, associations and interactions