ABSTRACT: With its 52 km2 areal extent, the Lake Köyceğiz is the 16th largest lake in Turkey. As it is indicated in the earlier studies, the lake is meromictic. Within the scope of this study, thermoprobe and CTD measurements were done in April covering 40 locations and spatial distribution of the lake water temperature and the areal distribution of the lake bottom sediments were investigated in detail. Investigations including electrical conductivity and pH measurements at water monitoring locations show that there are physically four distinct water bodies in the Lake Köyceğiz. The water and sediment temperatures in northern basin are slightly less than the southern basin. In addition, subaqueous springs increase the temperature of the water and sediment, and create a positive temperature anomaly around these locations.