This paper presents the results of physico-chemical analysis of the water, qualitative and quantitative analysis of phytoplankton and trophic state of the water supply reservoirs Garasi and Bukulja. The water samples were collected on water supply site, on different depths and in various seasons. The presence of six algal divisions with 194 taxa was established for both reservoirs. From the total number of taxa found, there were 162 taxa present in Garasi reservoir, and 134 in Bukulja. The highest number of taxa was established within divisions Bacillariophyta, Cyanobacteria and Chlorophyta. The results of physico-chemical testing of water, phytoplankton density and biomass, as well as chlorophyll-a concentration showed increased eutrophication in both reservoirs. Dominating blue-green algae are: Planktothrix agardhii, Aphanizomenon flos-aquae, Microcystis aeruginosa and Anabaena affinis. Detected large biomass of blue-green algae that can produce toxins dangerous for water lifeforms, requires appropriate environmental protection of these reservoirs. Based on the calculated trophic index (TSI), the Garasi reservoir could be considered as mesotrophic, while the Bukulja reservoir as mesotrophic-eutrophic model. (C) 2010 Elsevier B.V. All rights reserved.