Abstract: The aim of this study is that to investigate both ecological and numerical species assemblages of mollusc fauna of Lake Terkos (Istanbul, Turkey). In order to determine the mollusc fauna of Lake Terkos (Turkey) samples were collected from 5 stations monthly between February, 2008 to January, 2009. While a total of 11 mollusc species were found at the sampling stations, it was determined 817 individuals in per m² at average. Six species of them (Physella acuta, Galba truncatula, Radix labiata, Lymnaee stagnalis, Planorbis planorbis and Planorbarius corneus) belong to Pulmonota, two taxa of them (Viviparus viviparus costae and Bithynia tentaculata) belong to Prosobranchia and three species of them (Unio pictorum, Dreissena polymorpha and Anodonta cygnea) belong to Bivalvia. Furthermore in each samples water temperature, dissolved oxygen, pH, Ca, Mg, NO₂-N, NO₃-N and NH₃ measured at the sampling sites. Canonical Correspondence Analysis (CCA) explained 91.8% of the correlation between species and environmental variables. Water temperature (t), Dissolved Oxygen (DO), Ca, Mg and pH were most affective variables on the species absence.