We describe here the limnological characteristics of the Santillana Reservoir (Spain) and study the phytoplankton distributionduring 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 biomassin 1999 occurred between the end of summer and the beginning of autumn (September-October). Cyanobacteria, that weregenerally prevalent in phytoplankton community, clearly dominated during the peak of phytoplankton biomass. Microcystiswesenbergii was dominant amongst the cyanobacteria and represented almost 100 % of the biomass at the time of maximumphytoplankton value. The importance of environmental variables triggering cyanobacterial bloom events is discussed. Ourresults 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.