The objectives of this study were to determine: (1) whether or not heterotrophic microorganisms exert an effect on the trihalomethane (THM) or total organic halogen (TOX) formation potentials of algal extracellular products (ECP), and (2) whether or not diurnal cycling of THM and TOX precursors occurs in vitro. These objectives were addressed through culture studies of heterotrophic microorganisms and pure algae from both the green and blue-green divisions. Culture conditions were varied to study different aspects of algal and bacterial interactions. Results from continuous light assays indicated that heterotrophs, grown in the same culture vessel with algae, affected an increase in the potential of chlorinated culture filtrate to form THMs. No significant changes in TOX-formation potential were observed. Heterotrophs grown in volumes of algal ECP (cell-free filtrates of algal cultures) used the algal-produced organic carbon as their sole carbon source. These microorganisms further reduced both the dissolved organic carbon (DOC) concentration and the concentration of the DOC fraction that reacted to form TOXs. The fraction which produced THMs was not reduced. Heterotrophic microorganism metabolic activity increased the rate at which THMs fanned from chlorinated algal ECP but did not alter the seven-day THM-formation potentials. Their activity also increased the initial rate of TOX formation, but decreased the seven-day formation potential of these compounds. Attempts to demonstrate diurnal cycling of the concentration of THM precursors and other halogenated organic precursors, a phenomenon previously demonstrated in a eutrophic reservoir, were unsuccessful. / M.S.