The effects of ozone and chlorine on algae cells, extracellular organic matter(EOC), and coagulation ability were examined. Two algae were studied: a greenalgae (Scenedesmus quadricauda) and a diatom (Cyclotella sp). The results showedthat changes in the character of the algae cells and EOM from ozone yielded animprovement in coagulation ability of cells for Scenedesmus, but not forCyclotella. In addition, for both algae, preozonation increased the dissolvedorganic carbon concentration, which was difficult to remove by sedimentation.Furthermore, the EOM from Cyclotella caused a flocculation hindrance. Theseresults indicate differences in coagulation ability depending on the species ofalgae present. Includes 7 references, table, figures.