In the summer of 2010, the excessive growth of Cladophora laetevirens in the Southwestern shoreline of Istanbul, which carries great importance for tourism, drew major attention. The aim of this study was to investigate the excessive growth of Cladophora laetevirens, some ecological parameters, and the number of total coliform bacteria of surface waters in this region during the summer of 2010. During this period, this species multiplied and mucilage like environment was formed and the total coliform bacteria values were found to be between 105 and 108 MPN/100 mL. Due to the lack of available evidence prior to the sampling, it is possible that the excessive mucilage formation may have been trigged by Cladophora laetevirens. Cladophora samples were collected on different sampling dates during the 2010 summer (July and August). On these sampling days, the temperature, salinity and dissolved oxygen levels of the seawater ranged between 26.6-28°C, 14-33 ppt, and 8.2-9.7 mg/L, respectively. Nitrate+nitrite-N (0.21-0.26 μg-at/L), Ammonium-N (0.43-0.94 μg-at/L), Phosphate-P (0.53-3.50 µg-at/L), Silicate-Si (9.63-15.30 μg-at/L) and Chlorophyll a (3.68-14.11 µg/L) concentrations were measured. Marmara Denizi'nin Güneybatı İstanbul Sahilinde Cladophora laetevirens (Dillwyn) Kutzing ve enterik bakterilerin ani kitlesel artışı