Seasonal variation in the composition and concentration of epiphytic diatoms from a Karstic Lake, Great Lota, in Anatolia which has a distinctive ionic character was investigated during October 2000 and October 2001. A total of 66 diatom taxa belonging to 24 genera were identified. Mastogloia was the most dominant genus in all sampling periods. Gomphonema, Cymbella and Nitzschia were the subdominant epiphytic diatom genera. Relationships between epiphytic diatom assemblages and measured limnological variables were extracted by means of Redundancy Analysis (RDA). It was revealed that temperature, calcium (Ca2+), sulphate (SO42-) and total soluble phosphate (TSP) parameters accounted for a significant amount of the variation in the distribution of the diatom assemblages. While some dominant taxa (e.g. Mastogloia smithii and Nitzschia amphibia) appeared relatively abundant in the cold waters with high calcium concentration, other taxa (e.g. Rhopalodia gibberula and Cymbella affinis) showed affinities towards warmer water with low calcium level. Cluster analysis produced four major groups reflecting the importance of seasonal influence on the epiphytic taxa. Formation of these seasonal clusters was controlled mainly by the increase and decrease of Mastogloia species. Influence of seasons on species compositions was also assessed in terms of Diversity index (H') and evenness (J) values.