We describe here the limnological characteristics of the Santillana Reservoir (Spain) and study the phytoplankton distribution during a typical growing season (July to November). The reservoir is a eutrophic stratified waterbody in central part of Spain, in which the average phytoplankton biomass in summer is higher than 20 µg chlorophyll a 1-1. Maximum phytoplankton biomass in 1999 occurred between the end of summer and the beginning of autumn (September-October). Cyanobacteria, that were generally prevalent in phytoplankton community, clearly dominated during the peak of phytoplankton biomass. Microcystis wesenbergii was dominant amongst the cyanobacteria and represented almost 100 % of the biomass at the time of maximum phytoplankton value. The importance of environmental variables triggering cyanobacterial bloom events is discussed. Our results indicate that neither temperature nor N/P ratio are related to the bloom development. However, high nutrient concentration (ammonium in particular) and stability of the watercolumn seems to coincide with the maximum abundance of Microcystis wesenbergii.