The objective of this study is to assess the importance of algae and algogenic organic substances concerned with the formation of assimilable organic carbon (AOC) during algal growth and chlorination process. Laboratory tests were carried out using cultures of algal species (Phormidium tenue) with different culture medium, M-11 and CT. Extracellular organic matter (EOM) produced during the stationary phase and death phase of P. tenue (106 cells/mL) with M-11 culture medium led to significant increase of the AOC concentration, up to 100 times as high as its initial concentration. In case of CT culture medium containing a high DOC component, the AOC concentration did not increase significantly during P. tenue culture. The formation and removal of AOC derived from EOM and intercellular organic matter (IOM) by chlorination were also examined. The AOC concentration after chlorination of EOM and IOM became maximum with 2 mg/L of chlorine dosage while it decreased with increase of chlorine dosage when the dosage was higher than 2 mg/L, suggesting that necessary chlorine dosage to not form AOC is higher than 2 mg/L.