Abstract In June 1990, the City of Montréal opened a beach on an artificial lake in the southern sector of Ile Notre-Dame in the Saint Lawrence River, a man-made island built for Expo ’67. This lake is fed from the Saint Lawrence River and the beach stretches some 600m with a swimming area of approximately 15 000 m2. Since the desired environmental character was that of a natural lake, it was decided to avoid the use of conventional methods of water purification such as treatment with chlorine. Instead, a series of techniques more compatible with natural systems were chosen in order to maintain water quality. The major component of these techniques is a “filter-lake” complex. Occupying an area of 20 000 m2, the “filter-lake” complex consists of four ponds of varying depth where more than 100 000 aquatic plants of different native species were introduced. Intake pipes draw 28 litres/second of water into the first pond and then, water circulates by gravity through the three remaining ponds. During the last two seasons, several parameters were monitored in order to evaluate the efficiency of the system. The parameters considered were: soluble reactive phosphorous, total phosphorus, dissolved phosphorus, nitrates, ammonium, suspended solids and faecal coliforms. Preliminary results indicate that the system of artificial wetlands used at the beach of Ile Notre-Dame is efficient in water purification. The ponds are very good traps for soluble reactive phosphorus and nitrates. A good interception is also observed in the second pond for total phosphorus and suspended solids. Faecal coliforms removal however seems uncertain and long-term effectiveness is questionnable.