Three species of marine algae representing major taxonomic groups of phytoplankton, Isochrysis galbana (Chrysophyceae), Carteria sp. (Chlorophyceae), and Thalassiosira pseudonana (Bacillariphyceae), were utilized to investigate the potential of natural occurring chlorophyll a of living algae to produce trihalomethanes during the chlorination of saline waters. Chlorination of filtered natural estuarine water (salinity-23 p.p.t.) from the North Edisto River, South Carolina, results in rapid formation of 201-221 microgram/l trihalomethanes comprised mainly of bromoform (CHBr3) and chlorodibromomethane (CHBr2Cl). In the presence of 10 to the 6th power cells /ml Isochrysis galbana, chlorination of filtered estuarine water with sodium hypochlorite (NaOCl) to a nominal 10 mg/l chlorine increased the total trihalomethane concentration by an average of 41% (N=6). The presence of Thalassiosira pseudonana resulted in an average 24% decrease (N=6) while Carteria sp. did not produce a statistically significant effect upon the total trihalomethane concentration formed.