In June 1990 the City of Montreal opened a beach on an artificial lake where a wetland was built to maintain water quality. Occupying an area of 20 000 m2, the wetland consists of four ponds of varying depth where more than 100 000 aquatic plants of different native species were introduced. Intake pipes draw 140 1 sec−1 of water into the first pond and water then circulates by gravity through the three remaining ponds. During 1990 and 1991 several parameters were monitored in order to evaluate the efficiency of the system. The ponds proved to be very good traps for O-PO4 and NO3. A good interception of soluble phosphorus and suspended solids was also observed. Removal of faecal coliforms however, seems uncertain and long-term effectiveness is also uncertain.