During the summertime, water hyacinth, Eichhornia crassipes (Mart.) Solms, was produced on paper-industry effluent, in a 1 hectare lagoon, as a finishing treatment. This paper describes the evolution of the culture (cover area, biomass production). The available space was covered up to 60% within four months with an average production of 66.6 g d. wt d(-1) m(-2). At the beginning, the culture was restrained by the effluent foam and thus, an initial biomass of 100 kg fresh weight was necessary for its development. The lack of available space restrained the covering speed of the lagoon after 60% coverage. The harvest of the water hyacinths, 220 tons fresh weight, was carried out during the fifth month and was very difficult because of the entanglement of the plants. So, harvesting had to be carried out in order to reduce the plant cover from 50 to 25%, when harvesting was still easy but treatment was still efficient.