This project was carried out for one year in 1994-1995. Tajan River originated from Alborz Mountain and passes from sari city, and then entered into Caspian Sea. In this study 14 physical and chemical factors surveyed. This survey demonstrated that water quality in milk factory and kilka factory was more than standard. With utilization of water in agriculture period and dry of river, caused to increasing of chloride, total hardness, Ca^++, Mg^++ in estuary. This issue obtained to mixture of fresh water and saline water in estuary. In the survey of the river phytoplankton, 92 species identified that belong to 41 genus and five phyla. The Chrysophyta has more variation and density. Zoo plankton of river included in Protozoa, Rotaria, Cladocera and Copepoda, also the meroplankton such as Lamellibanchiate and Cirripedia observed. The high species variation is relative to protozoa but copepoda has high density and biomass. The five classes of zoobenthos observed in Tajan river such as Bivalvia, Crustacea, Arachnoids, Oligochaeta and insecta. The insecta class has six order and 22 families that Diptera order from insecta class has high variation. The river fishes are belonging to 23 species, 18 genus and eight families. The highest variation observed in station 4 in estuary that 17 species identified in this area. Ths seasonal stations situated in tributary river, station 5 (Zarem rood) has high variation and station 7 (Khormabad river) has important for salmo truta fario habitat
