Lakes are dynamic and stable aquatic ecosystems that are very important both ecologically and economically. Natural processes and human activities affect on the physical, chemical and biological characteristics of aquatic environments and they cause problems on water quality. Generally, most studies carried out on water quality was by physical and chemical methods. These methods show the water quality only in sampling time and they cannot predict the water quality over time, while the biological assessment is the useful method for conservation and biodiversity of aquatic ecosystems. In this study, the phytoplankton, zooplankton and benthos were used as indicator organisms for water quality in the Zayandehrud Dam Lake. The sampling was performed from different 4 stations during the spring, summer, autumn and winter in 2014 (with a 45 days interval period). For water quality measurments and phytoplankton community a 1 liter of water was collected by Nansen sampling bottle from the depth of 0.5 m, 5 m and near the bottom at each station. The zooplankton sampling was carried out by using a plankton net (mesh size of 50 µm) by vertical towing. Benthos samples at each station were collected by Ekman grab (20 cm×20 cm). Results showed that the range of temperature, pH, Ec, dissolved oxygen, NO 3 , NO 2 and PO 4 were 7-21.5 °C, 7.95-8.33, 221.6-358.8 µmohs/cm, 5.35-15, 0.79-1.87, 0.006-0.54 and 0.009-0.16 mg/l, respectively. The phytoplankton community comprised of Bacillariophyceae, Chlorophyceae, Cyanophyceae, Euglenophyceae, Dinophyceae and Chrysophyceae with the density range of 7211-157222, 5962-56770, 0-10437, 0-8291, 0-14583 and 0-11333 cell/l, respectively. The zooplankton community viz. Cladocera, Rotifera and Copepoda with the density range of 13-155, 21-141and 11-93 indi/m 3 , respectively. The benthos community was familes of Chironomideae (18-312 ind./m 2 ), Naididea (10-64 ind./m 2 ) and Tubificidae (314-1553 indi/m 2 ). The shannon-Wiener, Simpson and Margalef diversity indices were 1.42-1.86, 0.61-0.78 and 0.65-1.35 for the surface area; 1.47-1.93, 0.64-0.77 and 0.92-1.24 for middle area; and 1.55-1.84, 0.65-0.78 and 0.72-1.23 for bottom area. Generally, based on the findings of this research on water quality, phytoplankton community and esprcially the presence of genera such as Cyclotella and Dynobrion (as index of oligotrophic lakes) and also absent of Microsystis genus as important index of eutrophic lakes could be concluded that the Zayandehrud Dam Lake was Keywords: water quality, plankton, benthos, bioindicator, biodiversity