Chlorination applied in water treatment plants is possible to produce some harmful organic chlorine compounds, of a which trihalomethanes (THM) have been recognized as potential carcinogens It has been reported that algae could be potential THM precursors in chlorination process used for the disinfection of drinking water This study was performed to investigate the possibility of THM formation during the chlorination of water containing pure culture of algae For the experiments, two forms of blue green algae, Anabaena flos aquae f. flos aquae and Microcystis aeruginosa f aeruginosa were used. The results showed that THM formation is different between two different species and ozonation prior to chlorination can reduce the THM formation potential of algae up to 83 %