The Tahtali reservoir is the biggest surface freshwater to supply potable water to the city of Izmir. The capacity of the reservoir is 306 million m(3) and covers an area of 23 km(2). The aim of the study was to determine water quality and assess the trophic state of the reservoir. For this purpose, water quality of the reservoir was monitored from May 2006 to April 2007 on a monthly basis at three sampling points in the lake and at two in the creeks flowing into the lake. Water temperature, electrical conductivity, pH, transparency (Secchi disc) and dissolved oxygen were measured in situ and total hardness, nitrite-nitrogen, nitrate-nitrogen, ammonium-nitrogen, phosphate-phosphorus, silicate and chlorophyll-a values were determined in the laboratory. The effect of the creeks on the reservoir was obvious. Nutrients measured at the creek stations were found higher than those at the lake ones. However, nitrite and nitrate-nitrogen concentrations in the reservoir were below the potable water standards established by World Health Organization (WHO). The reservoir was found to be of first quality in terms of nitrate-nitrogen and ammonium-nitrogen concentrations under Turkish regulation of quality criteria for the freshwater resources. The results of the study showed that the trophic state of the Tahtali reservoir was oligotrophic based on Carlson's Trophic State Index.