The Medford Water Commission (MWC) of Medford, Oregon is in the process of adding a 45 million gallons per day (mgd) preozonation facility to its Robert A. Duff Water Treatment Plant (Duff WTP). In recent years the MWC has experienced summer taste and odor episodes that have been associated with algal byproducts, such as geosmin. Pilot testing to evaluate ozone for taste and odor control was undertaken to verify previous bench-scale testing results. The pilot consisted of three one-week testing periods that were conducted during August and September of 2000. The initial ozone demand and ozone decay rate at a given transferred ozone dose varied significantly due to the pH and temperature swings observed during the day. Taste and odor testing indicated that ozone was very effective at reducing geosmin levels in the spiked raw water. Testing done at 1.5 and 2.0 mg/L ozone dose resulted in virtually all samples being below the detection limit (0.5 ng/L). Testing done with ozone in combination with hydrogen peroxide did not show a significant improvement in geosmin removal when compared to ozone alone. The calcium thiosulfate testing showed that a dosing ratio (mg calcium thiosulfate/mg ozone) of approximately 1.6 is necessary to quench all of the ozone residual. Includes tables, figures.