Accumulation of heavy metals (Cd, Pb, Hg, Cr) in water and plankton of Sariyar Dam Lake (SDR) was seasonally studied from April, 2000 to December, 2004. In addition, seasonal changes in phytoplankton and zooplankton populations and species abundance were also determinate. Some physio-chemical parameters of water and their correlation with heavy metals were also determinated. The dominant phytoplanktonic algae determined were Choroococcus, Gomphospheria, Oscillatoria, Phormidium, Anabaena, Nostoc, Cystodinium, Dinobryon, Ankistrodesmus, Chlorella, Coelastrum, Oocystis, Pediastrum, Scenedesmus, Cosmarium, Oedogonium, Cyclotella, Achnanthes, Amphora, Anomoneis, Cocconeis, Cymbella, Diatoma, Diploneis, Fragilaria, Gomphonema, Navicula, Nitzschia, Nitzschia, Rhoicosphenia, Surirella and Synedra. The dominant zooplanktonic organisms determined were Asplachna priodonta, Brachionus angularis, Keratella quadrata, Keratella cochlearis, Keratella tropica, Polyarthra vulgaris, Proales decipiens, Bosmina longirostris, Ceriodaphnia quadrangula, Chydorus sphaericus, Daphnia longispina, Diaphanosoma brachyurum, Macrothrix laticornis, Acanthocyclops robustus and Acanthodiaptomus denticornis. High concentrations of all heavy metals were recorded in the plankton. Hg was lowest and Pb highest; but the concentration of each metal varied seasonally.