During the whole year 1998, samples of water have been collected twice each month in Lake Geneva between Hermance and Coppet. Qualitative and quantitative data have been recorded: dry weight, number of phytoplankton organisms per liter, diversity index, chlorophyll and total organic carbon content, turbidity and transparency. The organisms of the net plankton and the pumped water were listed. The results showing the seasonal changes are discussed and compared to those of the previous year.