The effects of preozonation on the treatment of water from the Seine River in Paris, France were evaluated in a pilot plant consisting of flocculation with basic aluminum polychloride (PAC) followed by sedimentation. Preozonation significantly increased the removals of turbidity, particle numbers and volumes, chemical oxygen demand, and ultraviolet (UV) absorbance at all treatment levels investigated, with the optimum ozone dose appearing to be about 0.8 mg/L. Treatment levels investigated were 0.3-1.2 mg/L for ozone and 15-60 mg/L PAC. The favorable effects of ozone increased with pH and the particle count of the untreated water. The increases in particle removals appeared to stem partly from increases in floc size and settling velocity. Includes 6 references, tables, figures.